This was requested so it's going into the review as well :P.
The Review -
First things first, the Warp Hunter is IA only so unless that is allowed at tournaments, it's not part and parcel of the standard Eldar army. It's got the standard heavy tank lay-out as well - AV12 side and front with rear AV10, BS3, fast, tank and skimmer with two weapon systems - one heavy & one twin-linked catapult. The defining feature is it's big old gun - D-cannon on steroids with special rules - which of course brings comparisons to the Fire Prism.
At 36" it's a lot shorter ranged than the Fire Prism but fits in with the general theme of the Eldar army but outside of anything AV10/11, it's the weakest platform around for Eldar so loses a bit of durability there. Beyond that, well the gun is pretty lethal against infantry with always wounding on 2's, instant death on 6's and an AP2 (which is actually good because the D-Cannon is a barrage blast). This is exemplified by having a template option which can be fired like a hellhound up to 6" away from the Hunter - aka more reliable infantry death.
Against tanks though, not so flash. The ranged option is a blast without any twin-linking so it's not going to be hitting too often and whilst it may seem like haywires on crack with double the penetration chance, doing no damage 33% of the time is pretty lacklusture. AP2 is also a bummer here as it means, well nothing. In the end the D-cannon is okay against heavy tanks as it bypasses their armor but against light tanks, aka the ones you see all the time, too inefficient. The template option removes the innaccuracy aspect but again needs to be quite close to come into play.
The usual upgrades are available to the Warp Hunter and the usual caveats apply. In the end though, the Warp Hunter is another one-gun tank which can poke infantry pretty well but is inefficient against vehicles.
Potential Uses -
In terms of Fire Prism or Warp Hunter, the Warp Hunter does have the edge in being able to ignore cover so that low AP comes into play. There's a lack of reliability when shooting at range though due to it being a single blast (and again, the tiny decrease in survivability) though obviously when the Hunter gets close the template can take over and kill things dead quite well. The Fire Prisms are better the more you take though and this brings their anti-tank and anti-infantry potential way up as you can start shooting from multiple angles, gain access to twin-linking, potential large blasts, get a profile which is quite scary against tanks if it hits, etc.
So which one? Both are good though I'd have to give the tiny nod to Fire Prisms in terms of the extra options they bring but, Warp Hunters are much better at working alone if their buddy dies or cannot operate that turn, etc. If they can get within template range as well, shooting up a tank means the contents aren't going to survive long so if you're able to protect the Warp Hunter and use them independetly, they will shine much brighter than the Prism. This is how they should be used which forces them forward quite a bit so screening to ensure they at least have a cover save is important.
Potential Changes -
AP1 assuming the bonus against vehicles stays or the ranged D-cannon shot should be more effective against tanks (i.e. non-blast or something). Otherwise having the D-Cannon always inflict a shaken result + whatever potential damage result on any vehicles hit would give it some actual suppression value until its anti-infantry capabilities can come into play. A minor decrease in points cost would also be beneficial but I'd say drop the Fire Prism more as the Warp Hunter has more potential and the same deal with one-gun tanks - a potential change in rules or how this is interacted with the individual model against weapon destroyed results.
The Warp Hunter is quite a nice vehicle in terms of being able to drop infantry and it can do so reliably with its template funciton - something Eldar can struggle with when they are only running mass S6. The issue is of course the range and the fact that the anti-tank capabilities of the Warp Hunter aren't too flash and are basically inefficient - much like the Fire Prism. If the Warp Hunter is included in the next incarnation of the Eldar codex, ensuring each one has a dominate role with alternate firing modes allowing for some duality within each individual unit would see both get tabletop time in differing lists. Otherwise, one is always going to be the superior and end up being seen all the time.
With this in mind I'd probably make the Warp Hunter the anti-infantry option with the flamer template (add the simple suppression bonus in) with the Fire Prism having the more dedicated anti-tank role (i.e. focused shot is two S9 AP2 shots, no blast; combined fire for two S10 AP1 shots, twin-linked, no blast & keep the large blast as is).