I've been thinking of them as basically the same thing. It's just the format that's different. So I think it comes down to what formats people prefer. The Google Groups have definitely died down lately, with only the odd conversation happening. Meanwhile, G2G has become a lot more active, and people are starting to use the project talk pages.
So, I guess that means I support making everything a Project. Though I still like the term User Group. :)
~Lianne