PostgreSQL is free. 100% free, for everyone to use. Even businesses.
MySQL, while it works for personal use, has some license agreements that make free use a problem. Not sure where or how, but they talk about it in appendix-D page 612 "The Winning Database" explains fully the advantages.
besides... if you know MySQL, then it shouldn't even be an issue. You should be able to look at what they are doing and draw the code up in your head. SQL translations are pretty basic. I use MySQL as well, though I will actually now be adding PostgreSQL as a second database (for redundant data, just in case). I mean, hell... why not. Its FREE! and once you have the basic SQL classes set up to use both it is no extra work for the rest of your programing. Not to mention it gives you an extra layer of Data validation. If they dont match, anywhere, for any reason, you know you got a problem.
"To err is human. To really F*** things up takes a computer"