I know this might come off wrong, but I see the 5 as just an upgraded 3. The 6 to me is in a field of its own since the styling and design work is not QUITE similar to any other BMW.
I do think though that the 6, to me, is a hybrid of a 7 and a M Coupe/Z, using the M Coupe/Z lines with 7 size.. But that's just my 2 cents
For the record I own a 3 as well, so i'm not bashing on them ^_^!
but 6's are still dead sexy!