I would stay and work it out if he was willing to put in the effort. I don't believe that an affair is grounds to give up on our family, it was a mistake and we are mature adults that can move past it.
I know many couples that worked through affairs and went on to have extremely happy marriages.
If he wanted to stay, I'd agree to work it out. I know he would do the same if it were the other way around. He was married before and they divorced because she cheated (she wanted the divorce, he wanted counseling) and he stills says he'd want to work it out if I did. I think we both feel that for something like that to take place, something would had to have been very wrong and needs to be taken care of.