I made this video for my friend comparing the differences between Melodyne Studio and Auto-Tune 5.
UPDATE: I've recently upgraded to Auto-Tune Evo, and the graphic mode is MUCH better then it was in Auto-Tune 5, and also the general GUI is a little different. I still like Melodyne better though for natural pitch correction.
As a general rule for me, if i'm tuning vocals in a solo acoustic song or a rock song or something, I'll use Melodyne because it sounds very natural, but if i want to tune vocals and have it be more of an effect, like in a rap song, I'll use Auto-Tune.
Sorry the HD doesn't work in this video, I messed up the setting when exporting it in ScreenFlow.