The problem is that very few people have actually used both MySQL and Pg in major performance-sensitive applications, so it's hard to get a good comparison. I have used Pg, but not as much as I've used Oracle and MySQL.
I can tell you a few things I like about MySQL, even though I can't really compare them to Pg in a meaningful way. I like how easy it is to administer. I like how fast the InnoDB storage engine is (faster than MyISAM for most of the work I do, even with transactions and foreign keys). I like the excellent documentation.
And finally, I like the fact that I know other people have done very serious work with it and succeeded. Yahoo, Google, Craig's List, LiveJournal... all of these companies use MySQL, and handle much more data than I need to. That gives me confidence that it can handle my work.