Personally, (assuming I didn't have a model release) I'd destroy the negs. I would never sell or give away negs -- they could reprint those things all day long, and there's not much you could do about it. No way. You could have prints of very poor quality floating around God knows where, all with your name attached to them, for God knows what purpose.
If you have a model release, the only thing you have to consider is that they will most certainly badmouth you at every opportunity. So, although legally you would be on safe ground, you may pay a price for keeping the negs. In this business, reputation is everything, particularly if you are working with nude models.
Just my two cents' worth.