When designing lists it's important to understand the different advantages the different build philosophies bring to the table. These build philosophies can be broadly differentiated into two camps - MSU with lots of little units or rock armies where fewer units are used but they are very powerful. Combinations of these two concepts are often applied to lists as well and ironically, this middle ground is generally what creates consistently winning armies. One of the key differences between these concepts is the purchasing of extra units or upgrades with spare points and an important balance needs to be struck. Spend too little points on the unit and it can end up useless. Spend too many and your unit (and army) can become inefficient.
More units -
In 5th edition the MSU list design style works very well. Whilst many point to the lack of major tournament wins by a true MSU list, the majority of strong lists out there are using the concept of lots of small units and maximum transport options. With the strength of vehicles in 5th edition and lots of armies sporting excellent transports with guns (Razorbacks, Night Scythes, Venoms/Raiders, etc.), being able to get as many of these on the board to supplement your other units is a good plan. This requires spending less on the units themselves to ensure the highest number of transports can be taken.
Following this approach will net an army with a high number of units/Kill points (20+ at 1750-2000 points) but gives the general a lot of options on the tabletop in terms of movement and ability to engage the enemy. The firepower of the army is also going to be quite high as minimum points are being spent on units to maximise the number of significant weapons brought to the board. Unit durability is low however as squads are often small - this is mitigated by multiple units but isn't a strong point of the army.
More upgrades -
On the other scale, we have beefed up units who are impossible to stop with any other one unit. This is the extreme side of this scale though is quite common with certain units (i.e. Nobz, Paladins, Seer Councils, Command Squads, etc.). Anything which has lots of options and is very durable can fit this roll pretty easily where they are very hard to take down and can hand out a beating. However, they are very expensive and are only one unit.
A more 'traditional' application of this philosophy is units above the minimum squad size with upgrades allowing them to do more than their smaller cousins. Examples include larger mainline Marine Troops, Kabalite Warriors, etc. Here, point efficiency and maximising the FoC by taking transports which operate outside of the FoC takes a back seat to making the units themselves more useful. They are tougher thanks to more bodies and can put out more damage, also thanks to more bodies but they can also gain significant advantages in terms of weapon loadouts (i.e. Dark Lance for Kabalite Warriors at 10-strong). Whilst you may not be running the most efficient choices and don't have as many transports, units like this provide the army with more stability.
More upgrades or more units? - Putting it all together
The question then becomes when designing a list - do you put more points into units or buy new units entirely? Buying new units is generally going to cost you more points if you follow the 1+1 principle and are looking to gain both a unit and a transport but is netting you a weaker base unit. If you don't spend enough points on that base unit, you're losing out on offensive potential (and I'll note here that this sometimes is useful such as Dire Avengers in Serpents or Acolytes in Razorbacks). Spending more points on the original unit can get you a unit which is more reliable at its job but if you spend too many points you start to become extremely inefficient.
There's a fine line to ensuring these philosophies are applied correctly though. If you go too far on the MSU side of things you'll end up with lots of little useless units and some nice transports whilst if you had one or two less units and forked out the points to get your original units some extra upgrades (i.e. meltaguns), they become useful. If you go too far on the rock side of things you'll have an army with only a handful of units and therefore a limited damage potential against smart opponents - you can only kill X units a turn.
The question you therefore have to ask is what do you want your list to be? If it's lots of little units for maximum firepower and tactical flexibility, that's fine but make sure you spend enough points to make the units useful. There are times when units hiding in transports are good tactical options and designing units to do this can be beneficial but you'll generally get more mileage out of your units if you're using both the transport and the unit as offensive threats. The same goes for the larger unit construct - whether you're going for a rock based list or simply not a MSU style list, you don't need to make every unit do everything. Duality is great but if you're spending 25 points to make a crap combat unit slightly less crap in combat, is it really worth it?
