Had the same dilemma a couple if months ago and went with the 650i Gran Coupe xDrive.
The M5 is an absolute beast and a legendary sport sedan but I fell in love with the Gran Coupe styling. The new N63TU with 445 hp is very powerful...more than enough to get you into trouble on a daily basis! If that's not enough you can go with a Dinan tune and take it north of 500hp and that should be available within months.
The insane power of the M5 would of been sweet but somewhat pointless unless you live in Germany with access to the autobahn. The 650i xDrive has plenty of power with the added benefit of all wheel drive which really keeps the power on the road.
Go with styling...I can't imagine anyone feeling the 650i lacking power!!!