For a long time now, most who’ve used both have felt that Postgres was a better database than MySQL, myself included. But until now, the stick that MySQL keep beating Postgres over the head with was performance. MySQL might be a toy database (okay, it’s moving away from that these days, but only in the last few versions and that’s not really enough dev time to be stable enough for critical stuff), but it was fast on cheap hardware, and for pragmatic web development, that meant it beat out Postgres every day of the week.
Basicly, for the first time in a seriouly major benchmark, postgres not only outperformed mysql, but it nearly matched oracle (which was running on hardware that cost more than twice as much).
There’s also this earlier post, which basicly showed that Mysql on a single-CPU machine starts off faster than Postgres, but falls off as the number of concurrent users
rises, until it’s way behind postgres, which remains pretty invariant as the number of users rises. It also shows that postgres takes advantage of multiple CPUs better than MySQL does (up to 16 CPUs anyway, where the postgres team say they still have work to do).
Something to think about. Do you expect to ever have more than one or two users on your site? Better start with postgres so, and save yourself the cost and hassle of a later switch (I don’t care how good your database abstraction layer is, no DAL is so good that you don’t have to change your app when you switch database. Maybe more on that later.)