Having used both on a Mac (where I mostly end up using the CLI anyway), I'd say that SourceTree is what you want to use for stuff like code review, whereas GitHub (other than local merges being hard to figure out - see my other comment) is quite neat and uncluttered - perfect for small projects or if your workflow revolves around their public repos.

(I don't see why this is being modded down - is it because I failed to mention I'm also using it in Windows?)