Comparing feature lists and screen shots probably will tell you more than I can, and giving both a test drive using the free trial versions of both. They both have a lot to offer those who want to do hand coding. Some people feel more comfortable in one over the other. I personally prefer BBEdit and have gotten very comfortable with it. It's been around since at least OS 7 so has had a lot time to mature whereas TextMate is still a young application.
Some of the big draws to BBEdit for myself are it's scriptability features so I can process documents using AppleScript, Python, Bash, etc. (anything you can run in Terminal), which allows you to add any missing feature you want. I also like the SVN integration, ability to handle a lot of documents and huge documents (1GB+ size; not that I need that very often). It's light weight and speedy.
I don't have anything against TextMate though. Test driving is your best bet because everyone is different.