I always vote Nurgle on Principal, but there is some advantage to it.
Khorne is great at melee, but against genestealers? If we are tanking them then we are using mecha to do so, otherwise we want to stall them for the artillery to do its thing. Survival>melee bonus and zero magic = lose.
Tzeench is nice on the surface, but that malus? We will be actively AVOIDING bonuses to our rolls. Tzeench the planner is right because we will be spending the whole time making impractical long-term designs that have no chance of coming our right the first time. I would rather have some action economy rather than getting one design for every three actions...
A.P. is nice, but slight, and flamers don't care. Mutations have lots of potential but unstable, and our malus will exploit that like crazy. Magic is the jive-bunny man, but again, it is innately warped, and for a god of planning there is way too much unpredictability here.
Nurgle: Slow and steady. Good at delaying tactics and resistant to the poor living conditions that are inevitable and whatever bioweapons the genestealers cook up. Slow is bad but bullets aren't "units" so who we can kill what we reach just fine. Not to mention the advantage of having our specialist units stick around for longer and our damage-sinks sink more damage...
Slaanesh is nice for speed, but doesn't apply to boolit, so don't kill stuff better. Extra manpower is nice, but then we burn it on our marines, so we basically have to just do nothing the first turns if we want an endgame. So Slaanesh is the prime choice for running away. Either an unstoppable cascade of limitless manpower or a self-negating benefit that amounts to nothing. No point...
Undivided adds nothing and loses nothing. Those vague bonuses and demons that can't be used are kind of impossible to exploit in any specific way. And then again, we CAN use demons in the deeper areas and they CAN stick around long enough to murder some mines.
Praise Nurgle, who gives us everything and takes nothing!