A lot of developers using .NET tend to go for Mercurial because a while back it felt a lot nicer to use on Windows. It's why I always preferred using Mercurial. A few .NET shops that use TFP/VSO are moving towards Git for the Visual Studio support, but I've noticed a few Python and PHP developers making the switch to Mercurial.
To be honest, I rarely need to do anything more than the basics so neither has a huge benefit for me. Neither feels particularly faster than the other, and both have comparable GUI tools. Aside from when I am pushing stuff to GitHub I tend to use whichever one pops to my head first on a project. I reckon a lot of developers are probably the same.