I use SourceTree as a Mercurial GUI on a Mac and it's been awesome. I bought it back then when you had to pay for it, and it was worth every penny. Developer behind the app is also super-responsive.
In my experience tortoise does not refresh its view as changes are made, so if you forget to push the refresh button, you may checkin too few files. Subjectively, tortoise also feels clunky compared to ST.
I would also much prefer to use the same client on Windows and Mac, as I'm using both.
