I've talked to some of my buddies who went through ATP for their CFI, CFII, and MEI and they said they don't feel nearly prepared enough to teach after getting their certificates. No one feels perfect when you get to Instructor, but they had hardly any experience teaching students. If you want to just make the airlines, then that's okay. But if you want to be a good instructor along the way, ATP might not be the best, unless you stay at ATP.