the biggest flaw in the 60D is that it does not have micro adjust AF
The 600D, 60D and the 7D all have the same sensor so there is no difference in image quality between them.
Wickedwombat makes a good point...microadjust is a VERY cool feature. On my 1D4 this function has brought more than one of my lenses back from virtual uselessness to highly useable. I wouldn't buy another body without it.
Which leads us to the 7D....do it if you have the $$.