Superficially, Necrons are almost as bad off as Sisters for Troop choices- they only get two choices and neither of them really have any unit options. (Technically Immortals can pick between two guns, but you'll almost always go with the same choice.) On top of that, the two choices almost might as well just be one choice with as similar as they are- you pay some points and upgrade to a better save and gun and that's about it.
However, that surface similarity is misleading- not only do Immortals and Warriors differ in a number of different ways, like their gun statlines, but the choices of transports and role in the army also vary. Moreover, their true customizability is hidden elsewhere, not listed in the unit entry but rather in the HQ slot wit the Royal Court.
Courts are the key to making your troops do what you want them to- we already talked a bunch about different roles the Court can help fill, so I won't go over them again, but it's good to keep in mind that you can cover weaknesses, open entirely new options, or just add some extra firepower to any of these squads by bringing a Cryptek (or, more rarely, a Lord) along. Think of those models like the Heavy/Special weapons troopers available in Imperial armies- while the basic dorks in a squad might define its broad abilities, your choice of Crypteks will tell you what the squad is actually going to be good at, whether tank-busting or murdering infantry, slowing the enemy or shutting down tanks.
I'll try to compare and contrast the facets of these two units that make them different from each other in important ways- obviously we know that both of them are largely similar, but as always the devil is in the details.
Warriors
Warriors are the durability-focused selection for a Necron army; though they are technically easier to kill than Immortals, body for body, their significantly cheaper price (a bit more than 2/3 the cost) makes them able to be taken in larger numbers as a result. Their cheapness can also be a virtue by itself- for small five-man squads meant only to score the backfield objectives, Warriors may be a prime choice.
Gauss weaponry is your only option for a Warrior squad, which means mobility will be limited if you want to actually shoot with the squad members (as opposed to just the attached Cryptek.) A Phaeron Overlord can help you ignore this, but given the limited number of such characters you'll have in an army, you obviously can't rely on it to keep all of those units mobile. Gauss is a blessing, on the other hand, for glancing tanks, where the weight of shots (and actual effect of the Gauss rule, on heavier targets) can really start to annoy the opponent.
Warriors will be your pick when you're looking to diversify your threats against heavier tanks, when your other units will be pulling most of a list's weight (leaving the troops free to score objectives and little else), when you're on a tight budget, or when you're running a Phaeron Overlord and want to give him a good place to sit. They mesh well with Immortals when used to sit a backfield point or when providing cover in their Ark.
Immortals
In contrast to Warriors, Immortals come with that almost-MEQ statline, gaining the upgrade to 3+ save but not I4. At first glance one would assume this makes them the tougher of the two choices, but the reality is that they're actually the shootier option as well as the mobile one; point-for-point, Immortals put out significantly more firepower than Warriors do and, thanks to Tesla, are fully effective on the move- an important quality in an army with middling range and a dislike of melee combat.
Immortals also get an option to pick two different guns, unlike Warriors. Both guns are 24" and S5, with Gauss being Rapid Fire and Tesla being Assault 1. This makes Tesla the default choice, as it's not only significantly more effective in the 13"-24" zone where you'll do most of your shooting but also allows you to advance and retreat without requiring an Overlord to hang with you. Gauss has uses, but for the most part Warriors are a better choice for Gauss platforms- their superior numbers make better use of the "glance on a six" ability, and against vehicles S5 will most commonly just glance tanks on a six anyways, so the stat upgrade is a bit wasted.
Immortals will generally be your pick if you are going for a foot or hybrid type of list; their ability to rove about freely makes them much more useful than Warriors in that regard. They also can play MSU better despite being more expensive, as the ability to act independently from their transport allows you to effectively double the number of targets the enemy has to deal with.
Ghost Ark
This is the other half of the Warrior survivability equation- where Immortals are forced to mostly hoof it, Warriors can hide inside their AV13 unshakeable transport and shoot out the top, Cryptek included. This usually gives them a better vantage point, better protection from the torrent guns that can be dangerous to their 4+ save otherwise and even the potential to add members to the squad, on the rare occasions that you get out before it's wrecked.
The Ghost Ark itself is rather pricey at 115pts, but AV13, Living Metal, the ability to repair squads, open-topped status and two arrays of guns that can fire independently to provide suppression or anti-infantry as needed is actually a quite good suite of gear overall. The large profile makes it difficult to get cover, but easy to provide cover for other models (depending on what you build for Night Scythes, of course.)
Any time you're taking a Warrior squad, you'll want to strongly consider giving them an Ark as well- that's one of their main strengths. Smaller squads may pass the opportunity up, but larger ones have little reason not to go the extra mile and get a really good place for guys to sit and shoot from. Arks can also be "stolen" by members of a Court that, for whatever reason, wants to stay together- Lords will appreciate an open-topped transport to assault out of and Crypteks appreciate the extra protection to keep them shooting longer.
Night Scythe
This is an interesting transport because it has some major divergences from what we might regard as the standard Necron template. It's a Fast vehicle and comes with a large transport capacity that can hold some extra unit types, but if the vehicle is wrecked it kicks things back to reserve, which is a major downside in most cases because it means you're walking in from a board edge- probably not where you wanted the unit to be. A twin-linked Tesla Destructor (because there isn't any other kind) gives it some serious firepower, and the 24" range isn't really an issue on such a maneuverable platform.
Despite the superficial similarities, Night Scythes are quite unlike the Dark Eldar vehicles they resemble- lack of open-topped, bump to AV11, and Living Metal makes them actually more survivable than a Rhino is, generally speaking, so they might more properly be compared to a Blood Angels Razorback or the like. Night Scythes will generally run "empty," with their owning squad simply waltzing around the field to avoid getting dumped back into reserves (and to allow them to shoot.) In some situations you may want to start onboard and zoom forward to deploy out, but mostly the Scythe is acting as an independent vehicle.
Night Scythes are the natural (and only) choice of transport for Immortals- unless you're building a foot-only list, you'll probably want to take one for every squad that can, as they are excellent for the points. On Warriors they are technically an option, but as a rule if you're taking Warriors the Scythe is not going to be great- they can't as easily keep up with the battle line while firing when compared to Immortals and aren't as able to handle a variety of targets effectively. A mix of the two vehicles, on the other hand, is still very viable option- Ghost Arks can provide mobile cover for Night Scythes, who can range about to hit elusive targets.
Conclusions
Neither Warriors nor Immortals, Arks nor Scythes are strictly superior to the other- both of them have significant advantages we need to consider when building a list. There are some common builds and themes you will tend to see in lists, though, and I think those are worth looking at in terms of what they achieve.
-5 Immortals (Tesla), Harbinger of Destruction, Night Scythe
This is the MSU option for Necrons, equivalent to the "five Marines with melta in a LasPlas" unit, but better in its own ways. You have a two units that can both put out significant firepower and neither of which is easy to destroy, but neither of which is an easy/obvious target, either. The two can advance up the field shooting and rely on Night Fight or cover to protect them. Where possible, a second Harbinger is added to the squad.
-5 Warriors, Harbinger, Ghost Ark
You have a choice of several Harbingers that can be effective here (Destruction, Storm, Despair, Transmorgrification), each with its own potential uses. This squad doesn't particularly take advantage of the potential resilience of the Ark's ability to repair a squad, but it will occasionally come up.
-9/15 Warriors, Harbinger of Destruction, Ghost Ark
This is a squad that will usually be babysat by a Phaeron and have some portion of a Court stuck inside its Ark, but it can also take advantage of other, nearby Arks to keep "powered up", making it an excellent choice in hybrid lists. Lacking the protection of an Ark, most Harbingers will have trouble finding effective use, with Destruction (and occasionally Transmorgrification) being the only real choice. The smaller version can hide inside the Ark when needed and is better in smaller games; in large games the oversize squad will tend to fare better, as it can weather more firepower and doesn't take as huge a portion of your total points.
-5 Warriors
This is going to be your classic backfield scoring unit- dirt cheap, reasonably tough to bring down, and 100% expendable. You normally won't bother putting a Harbinger in it because you expect to be spending most of the game in reserves, Gone to Ground, or out of range.
Large squads of Immortals will only rarely be seen in good lists, as Necrons have so many quality choices scattered across their FoC slots there is little need to max out on six Troops AND to pump those squads to maximum size.
Some lists will want to run only one of these squads (commonly the Immortals or Warriors in a transport) multiplied many times over, but others will happily mix-and-match them to suit their needs- perhaps 2-3 Immortals for forward threat, 1-2 Warriors in Arks to more securely hold midfield and a minimum Warrior squad to hang on a home objective.
