IMO the silver (light grey) leather option in the M5 looks stunning with the contrasting panels in the doors and seats with black carpet and seatbacks. I think the M3's interior with P.S., NON-extended leather will look as good so that's what I'm going with!
Grey interior will be fine with a grey exterior if you're worried about resale or appeal. I see it all the time on 5 series cars and even on E46 M3's....