I have stuck with MySQL primarily because I'm used to it. I started with mSQL, which lacks some vital features, so when I found MySQL I thought it was great. For simple databases, MySQL is easy to use and learn. Postgres has more advanced features that compete with the "big boys", but I've never needed them for the kind of work I do. I considered switching once, in case of future need, but I didn't like the way Postgresql handled things like OIDs and stuff as opposed to MySQL. I'm sure if I had started out with Postgres I'd swear by it instead.