I used to use Windows, SVN, Tortoise SVN, and Beyond Compare. It was a great combination for doing code reviews.
Now I use OSX and Git. I've managed to kludge together a bash script along with Gitx and DiffMerge to come up with a barely acceptable solution.
I've muddled along with this setup, and similar ones, for over a year. I've also tried using the Github diff viewer and the Gitx diff viewer, so it's not like I've not given them a chance.
There are so many smart people doing great stuff with Git. Why not the side-by-side diff with the option of seeing the entire file? With people who have used both, I've never heard of anyone that likes the single +/- view better, at least for more than a quick check.