Really I think it comes down to 2 things.
1. How much of a hardcore gamer you are
2. How competitive you are.
If you are on the high end of both of these, you will likely wish to master all, if not, I would say you would likely play all a bit but then find that single or 2 divisions that falls into your playstyle, which everyone has and always reverts to in games.
For example, I personally love light infantry/vehicles, and so I will fall into that gamestyle, so 6th Airborne suits me perfectly with the tetrarchs and recon vehicles, meanwhile I have tried guards armoured a bit because it has 1 of my favoured "light vehicles" the AEC(and is the only division with it), but sadly the division doesnt seem to suit me as of yet, as though on paper the cromwells are simular, that asthetic appeal isnt there to me.
But given that I tend to keep to 10v10 right now due to a lack of time to "git gud", my views of the game are rather, skewed towards that and how i play there, for example with 6AB i could pretty handedly end a game on the enemy side of the map still with about 5-6k kill 1800 death in points, guards I can "hold my own" in the middle with either even losses or suffer losses heavily, something like 5k losses for 1800 kills, but I hold my ground well enough, which in conquest is what you aim to do. Just need practice i guess.
So yeah, in the end, i preff the light infantry 6 AB myself as of right now.