SourceTree went downhill after the last major UI overhaul and never really recovered. I used to love it for more involved commits (picking apart lines and hunks, etc), but now it's slow and clunky, even on nice hardware.
The main thing I like about SourceTree is that it makes it easy to stage 'hunks', or select lines to stage. I also like that (as with UIs in general) it makes it a bit harder to make mistakes (by, for example, typing the wrong flag on the command line). But it's become unbearably slow. I really wish they'd fix this.
Have you tried [Git Cola](https://git-cola.github.io/)? It has support for staging chunks/selections.