You also have to look at the how complex your database will end up being. mysql is excellent for sites that do a lot of lookups but relativily little database posting. It's great for shopping cart type sites.
However, if you have many interconnected tables that are constantly being updated then things like transactions and stored procedures become pretty handy. And having nested select statements would be really nice.
mysql 4.x will start to address some of these issues but it's still still in beta.