I've used both for similar amounts of time at this point. I know a _lot_ about the implementations details of MySQL, and a helluva lot less about PostgreSQL, and that's because I hit _way_ more landmines with MySQL. It's just so easy to use it the wrong way and get saddled with ugly problems for months or years.

To be fair, my PostgreSQL experience is more recent, and both products have matured a bunch, but I've also seen plenty of the bad old MySQL problems—like MyISAM tables and 3-byte unicode—in just the last couple of years.