Personall, id get a warranty, regardless which of the 2 you choose, but that is up to you. Im sure the 6 would be more preactical, so how often would you need to carry more than one passenger in comfort? I think its a beautiful shape, and im still a fan of the original Bangle 7 series. (Its an interesting car to look at)
Personally, if you want something to replace the Zed, i think you will find it hard to choose the 6 over the 911, but if you are willing to compromise slightly on driving thrills (not in a straight line, as lets be honest, you would never get bored of that V10) then id go for the 6. The 911 is more outright sports car with a touch of practicality added in with the rear 'seats'
Have you thought about an AMG Merc, CLK63 or something like that?