My problem is choosing which OSS DB to help with. MySQL is the most popular, so getting involved with it would be most helpful to my employment prospects. But its list of fundamental design flaws seems so severe that I can't respect it as a database.
I'm attracted to the robust correctness requirements of PostgreSQL, but there don't seem to be many prospective employers using is. So while I'd enjoy working on it, I don't think it would be very helpful to my employment prospects.