If you are thinking about the different Septs, here's a rough breakdown:
T'au: Favours specialist units like Hammerheads that have low volume of high powered shots. This gets the most bang for their buck out of the 1 re-roll to hit and wound per unit. Their sept strat does still support some smaller massed fire though too, with a strat that gives a +1 to wound a against a unit that's already been wounded. Overall, very good at focusing down key targets / elite heavy armies like Custodes, but weaker to swarm armies.
Vior'la: I think these guys are the weakest, they read like a close range focused Sept, but Farsight Enclaves just do it so much better. They can recycle
CP pretty well though.
Sa'Cea: These guys are the big gun line Sept. Vehicles and Infantry are hard to hit at range with the benefit of Dense Cover at 18" and 12" respectively. Put the infantry in light cover and they get really hard to shoot off the table. They also have some passable mitigation when the enemy closes into melee. Kings of the castle for sure.
Dal'yth: This is the Sept I play the most and is the Auxiliary focused Sept, providing benefits to your Kroot and Vespid while still giving some extra armour to your other Infantry and suits. I use this with masses of Kroot to flood the midfield with bodies and throw out an ocean of small arms fire to take down anything below T8 with relative ease. Then just hold onto the objectives as you're pretty hard to shift and you have an ocean of obsec bodies. It's good for suit focused armies as well as it gives them (and non-auxiliary infantry) permanent light cover.
Bork'an: Another ranged focused faction, however
Sa'Cea does most of it better. The -1S to attacks against vehicles and suits would be good, but since it's only for weapons S7 and under, it turns out to be pretty situational. (Most of the anti-armour is S8 or better so it really only protects you from massed light shooting). The pros are that it makes your commander VERY shooty. I could see a Coldstar with all burst cannons being really deadly with the
WT and Sig System. (An extra pip of
AP to all weapons, 6's to wound do extra mortals, and +1 to wound). The other nice thing is the Strat that lets one of your attacks ignore an invul. Feels like Bork'an is a hybrid of T'au and
Sa'cea, decent at single target focus and with some ranged defense but nowhere near as specialized as either of these two.
Farsight Enclaves: These are the others I have the most personal experience with. They are the up close and in your face faction. You always want to be within 12" of your enemy and can then forgo markerlights on most of your army. Provides good shock and awe attack with Deep Striking Crisis Suits re-rolling hits and wounds and gives you a pretty killy warlord.
Beyond that, there are the custom Septs of every flavour, but none of them really stand out to me as that great unless you are dedicating your entire army to doing something very specific.
Hope that helps out a bit.