[JaydedOne]

I'd take Mind Burn, simply because it doesn't run out of ammunition. Keep in mind that illithid, if set to fire, do so until they're out of ammo and then charge blindly (stupidly?) into battle, regardless of whether they're wearing armor or not. In long, drawn-out battles, this is a major minus as your commanders get drawn into hand-to-hand when they'd be a lot better staying out of the fray 9 times out of 10.

Then again, you asked about early battles and, as such, you may not often be in situations where you're running out of ammo. Mind Burn kills more quickly, but I don't think it has the same stopping power on a turn-by-turn basis as Mind BLast.