Yeah, Ubuntu is nice, Mint is an offshoot of that and one of the most popular distros right now (#1 according to DistroWatch). I'm using it and I'm happy. It's quite user friendly and easy to work with. Steam has full support on Ubuntu based distros but I'm not sure where else it's supported (I think it's on Fedora, too).
The good thing about Mint is that if you don't know how to do something in Mint and you can't find information online about Mint, you can just substitute your search with Ubuntu and you'll still get the solution as they are pretty much the same when it comes to that.
If you have any questions, come over to the Linux usergroup [http://www.escapistmagazine.com/groups/view/Escapist-Linux-Users].