From the experiences from my own team mates, who started by using a GUI for Git - it took them a lot longer to understand the concept of Git and they way it works than the team mates who worked from the CLI.
I'd suggest that you spend just that little extra time learning Git on the command line and I bet you'd be more efficient in the future - especially when you start chaining your commands. It's much quicker :)
But if you really want to work with a GUI, then SourceTree seems to be the favourite.