My personal experience is that postgres is much nicer to work with than mysql, from a developer perspective. It seems like whatever you want to do, you just read the documentation and do it whereas with mysql it's like whatever you want to do, there's something close if you don't mind changing a bit and being really careful about how you do it. However, I've got no more experience than playing around with both at home for a while (I did do a school project with mysql and wasn't very pleased). But obviously there are lots of people who get on better with it than I have, or they wouldn't use it so much.
I never really cared about performance, since I didn't do anything serious with them. Price is roughly the same.