Yep, mercurial is great, much better design at the API level, though I don't know the internals of either of them. Fig is pretty good but it feels a lot slower than git; I think it's doing more stuff remotely than I expect it to.
If you like Fig and want a similar (but better, IMO) UX that can be used with Git repos, you may want to try my project: https://github.com/martinvonz/jj. It's also pretty fast (can rebase >1k commits in <1s in the Git source repo, for example).
I'm on the Fig team at Google, but the project is my own.
