That aside, I've used both over the last 3 years (2 in git; 1 in hg) and although I prefer git, hg is a fantastic tool. I think the largest difference between the 2 is usability. I do believe hg is quite a bit easier to pick up. It does a really good job of, by default, making sure you don't shoot yourself in the foot and also keeps you further away from the 'nitty-gritty'. Git, on the other hand, allows you to do whatever you want and generally makes the assumption you always know what you're doing. The details are not very far down from the surface and it's easy to get yourself in trouble if you don't know what you're doing.
Personally I prefer git because I feel so close to the metal, but there's certainly a perfectly valid argument to choose hg over git.