SQL Server Express vs. Oracle Express Edition vs DB2 Express-C
I think you are missing couple important parameters. If you believe the project will advance , you have to keep in mind that one day you will need to switch to non-express version. And at that point license cost becomes very important; switching to another RDMS may require serious refactoring (and usually it does). For startup project I'd not pay too much attention to hardware configuration supported in express version, I'm pretty sure even the lowest will work fine. More important thing in my opinion is DBA's skills; in practice, not so many database professionals are equally good in all RDMS. So ask your DBA which server she/he would prefer before making a decision.