P4Merge is definitely one of the best diff and merge tools around. Totally love it although it comes with a 44 megebytes overhead.
Go ahead and download it from here. Then fire up your sourcetree and goto Tools -> Options -> select Diff tab. This is where you can configure P4Merge as your Sourcetree default merge / diff tool.
After you have installed P4Merge, you can just select from the drop down box highlighted above.