Personally, I'd opt for the mice because they have a shorter generation time, are easier to handle, and I don't feel bad about killing them. In a 30 long you could have quite a colony of mice, far more than your snakes will be able to keep up with. I've left the males in and taken them out when breeding mice and rats (not for my snakes). It actually depends a lot on the size of the colony and the personalties of the animals (how much your particular males seem to fight). Some males will kill their own offspring, but they seem to fight/kill each other less when there are females around. For a smaller colony I'd probably only have one (maybe two) male and I'd take him out not because I'd worry about him hurting the offspring, but because I'd worry about overpopulation. I've never had a separate tank for rearing the young, but I do always try to set aside an area (with nestles or one of those plastic igloo things) where they can hide their young. The 30 gallon is going to be a lot to clean though especially because it is much larger than you will need for the number of mice you want.