Earlier quoted context omitted.
I was on the team of a large org doing a POC between the two. Merc did some stuff better, git did more. I am glad git won out overall. I think the only feature Merc had that I cared about was tracking folders as objects.
Mind if I ask what Git did better? The main selling point for the difference between them was the Linus, Linux and then the whole community chose Git. The only big difference I recall Git winning on was in third party tooling and Git supporting more than two parents merging.
1) We used some super crappy Clearcase version control system. It was easier to layer Git on top, letting us have a VC on top of a VC. I can't remember if it was impossible, or just easier to do in Git.
2) Conflicts did better. We made kind of torture test of merges and put both systems through. Git was able to resolve more out of the box.
3) Speed of operations, I think Git was a bit faster out of the box.
None of this was about Github, as it was comparing self hosted HG vs self hoted GIT.