If your motivation is reducing human suffering then the two painless deaths are preferable. If your motivation is saving the most lives, then one painful death wins out. It's a tricky one because if reducing human suffering is your primary objective, it leads to the inevitable conclusion that the best option to reduce human suffering is to never exist in the first place, which isn't a very humanist conclusion. I think ethically I'd have to go for the one painful death then, but it would be hard to live with. My heart is telling me the two painless deaths would play on my conscience less.