I used SourceTree before, but now I prefer Git Extensions.
As a note I don't have much experience in git GUIs, I only used it at the start of my career (10yrs) with SVN/Git Tortoise, and SourceTree a few years ago when doing non-web development on Mac.
But I had to train non-tech team members (GFX,QA) to use git and SourceTree was a good friendly way to achieve that.
For Developers I recommend using the BASH, this all mighty tool that fixes everything! Except for conflicts and few other heavy operations :).
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