If oneshot probability is the main factor, higher min dmg (for the same average) isn't always better.
It actually all depends on the mob's life pool, which is hard to control anyway (as a variable):
life : 2
oneshot proba - raw dmg 1-5 : 80%
oneshot proba - raw dmg 2-4 : 100%
life : 5
oneshot proba - raw dmg 1-5 : 20%
oneshot proba - raw dmg 2-4 : 0%
Interestingly, if life = avg dmg (exactly), higher min dmg can be better:
life : 3
oneshot proba - raw dmg 1-5 : 60%
oneshot proba - raw dmg 2-4 : 67% (up-rounded)
I think whats more important than the oneshot probability is the fact that for leaching purposes, sometimes it's necessary to do high dmg to basically refill that life, for instance against ubers. For an extreme example, if your damage is like 1-10k, and you roll 1 twice, you can get deleted without significant life leach as you get hit the second or third time. If your damage is like 4999-5001, even for the same avg damage, we can all see how that allows you to consistent leach your life back between hits from an uber