What's the single biggest consideration you should make before considering upgrades, war-gear and other bonuses... unit size.
I'm not promoting this as a game breaker, genius cunning plan or anything ... but unit size is far more important than we frequently think. The formula is very straightforward. Always deploy the unit at the following size (if possible):
4t +1 (where T is the multiple of the unit).
The thinking behind this is simple ... and yet ignored by many, many players. Lots of units come with variable unit sizes, Space Marines can usually have 5-10, Tyranids or Orks 10-30 or Guard with up to 66 models in a single squad. So what numbers best?
The simple answer can be found in looking at the leadership test. A break test is taken when you take 25% casualties. Therefore 4 men will have to test when they get 1 casualty. Whereas 5 men require 2 - that's 4t + 1 in effect.
Example 1 - Tau Fire Warriors
I deploy 3 squads of 12 firewarriors. A quarter of 12 is 3. However if I field the warrior in squads of 9 (where T = 2, 4t + 1), their breakpoint is still 3 but I'm able to field 4 squads of 9 firewarriors.
Example 2 - Imperial Guard
A combined squad of 20 men has the breakpoint of 5. If I attach a priest, that breakpoint increases to 6. A commissar would significantly change this again, increasing the leadership AND giving you a re-roll (after he's shot the sergeant!)
So the next time you're wondering when to stop adding models, the answers simple: 4t + 1.
Disclaimer
Obviously, 4t + 1 doesn't count in every (or even most) cases. There is an advantage to having 10 tactical marines over 9 (you get free special weapons), there's an advantage of having 12 firewarriors in a devilfish (or even 6 - depending on your tactics), and the advantage of having 30 ork boys is fearlessness and close combat impact.
For some species (Tyranids for example), size of unit is irrelevant as they are mostly fearless.
But in those small cases (like with Tau or Guard or Orks or Elsewhere) t4 + 1 will give you the maximum durability at the minimum points cost. Imagine deploying this across multiple Kroot, tau and crisis and stealth suit teams (which can easily be 5 man or nine man, thanks to gun-drones) ... suddenly you fire power's going to trigger a lot fewer leadership tests...
(ED: Which is why I use mortars.)