If you perfer Mints GUI that's probably because you are not digging the new Unity in Ubuntu right? I hear ya.
I rolled back to Ubuntu 10.10 to get my good old GNOME back.
The deal with Mint is that IT IS UBUNTU. When a new Ubuntu comes out, they tweak stuff to the way they like it an release mint a little while later. with no ubuntu there's no mint.
This means that every software package that is available in ubuntu is available in mint. So why mint then?
Well for one, if you like the GUi better, then hey, there's a plus. But mainly I like Mints philosophy that they aren't going to fix what ain't broken.
When ubuntu went to Unity a lot of people swtarted looking for a solution. Sure for now you can switch to Ubuntu Classic (GNOME) but I found it to be buggy - hence my retreat to 10.10.
This mentality is used by Ubuntu in other places as well. Like some people will have 10.10 and everything is working, then they come out with 11.04 and (besides the desktop) they blow something up. This is something they actually expect and accept. It seems to me that stuff should be releated in beta and tested fully before acting like it's ready for full public release.
You just have to look at their forums when a new versions comes out. People lose wifi, or video, or sound and ubuntu acts like "ok, there's a bug, we'll fix it next time" when it never should have been broken in the first place.
This is where Mint really shines. Their model is "if it ain't broke, don't fix it". they don't just plow forward hoping everything works out with a cavalier attitude like "we'll fix it later". That's not what I expect in a stable friendly desktop environment. Seems kinda stupid if you want to get the average everyday computer user to use your OS.
Also, I kinda like the mint desktop too but I like the bar across the top and bottom, not just the bottom so for that reason I haven't fully commited to mint. I'm sure I could probably add it too, but I haven't messed around with it too much.
So if you like Ubuntu as a nice solid base with lots of software available and pretty decent support but don't want to risk losing functionality due to an upgrade that hasn't been fully tested yet - Mint is the winner.
If you want to be bleeding edge - ubuntu is the way to go.