I had the opposite experience with SourceTree going from Windows to OSx. The lack of tabs on the OSx version is horrible.
But the UI aside, the problem I was having is that it would just entirely hang the app if you had a large number of changed files or large diffs. This was about three months ago, so it's possible the performance problems have been fixed.
Haven't found a good alternative on Windows for a Git GUI tool that comes with Git-Flow.