As a general "rule of thumb" The real differences between the two are going to come down to the following
- Redhat focus' on older, more thoroughly tested packages, as such it has rock solid up time.
- Ubuntu focus' on more up to date packages, as a result you will generally get access to new features far more quickly.
From a database standpoint. I generally would consider RHEL to be a better option, as "features" aren't something that come out frequently, and stability is more of an issue. That isn't to say ubuntu isn't "stable", just that "stable" is redhat's whole bag. Its what they shoot for.
The other point to consider is "ease of use" and "comfort." If you are more comfortable with a debian based system, then that is hard to put a price tag on, ubuntu may be the choice to go with, the reverse is also true. If you have no personal preference, then I almost always go with whatever I find a more robust and thorough community for surrounding what I'm doing. If you don't have a preference, but there is information overflowing about how to do what you need to do on RHEL, Then honestly RHEL is what you should do. The hours you save yourself in troubleshooting in those cases is invaluable.