I almost exclusively use Textmate, but to be fair the decision to switch to Textmate (coming from codewarrior in OS 9 days), was mainly because the XCode editor (then named ProjectBuilder) was annoyingly slow at editing larger files.
I'm sure that changed a lot over the years, but I see no reason to switch so I don't.
That said, one thing where XCode really outshines Textmate is the integration of a debugger, when you're just starting to learn, I think having an integrated debugger will help you quickly understand what's going on with your code, and will be a great advantage...
If you've got Textmate, just try em both for a few months, you're a student anyway, you should have plenty of time
One thing to think about is that you do have the opportunity to dig in and learn more than usual since you are in school. If you don't have much programming experience, I would go the xcode route, but it would ultimately suit you better to learn how to use gdb using a text editor from the command line. Do yourself a favor and don't limit yourself to an IDE for the rest of your life.