MySQL is free, is fast, can handle really big tables, is easy to use and you can find documentation and help extremely easy.
I also know MySQL better than say PostgreeSQLPostgreSQL and that is why I choosechose that one for my persistent multiplayer game.
Go with what you know best and if the worst case scenario happens (everyone wantwants to play your game and the DB knuckelsknuckles) you can always change (or buy a faster server).