I'm a new user in webfaction and have some experience writing python web apps mostly in mediatemple servers, although I'm not an expert in server administration. I've always used MySQL as my data provider. Reading the WebFaction documentation, the choice for database engine in the tutorial is postgreSQL. Is any restriction about using any other database engine?.
No, not at all. I have apps running on WebFaction that use both to the same effect. In some ~2 years that I've been a WebFaction user, I haven't encountered anyone that suggested that one or the other has less load/yields better performances and that certainly hasn't been the case in my environment. So make it a choice of personal preferences or project requirements which of the two you take.
If you still have any concerns, you can ask them on the WebFaction Community page. The staff members are always happy to exhaustively address any of your questions in full detail. It's a SO Q&A clone, so you should be feeling right at home.