The M5 is a 500-horsepower beast that is supposed to put you back into your seat when you step on the gas. The 650i is a true luxury cruiser with questionable styling.
Both come with their own set of built-in quirks, mainly the damnable iDrive system … a mouse-controlled system mounted on the center console. You’re supposed to be able to adjust the navigation, audio, entertainment and climate controls with the silver mouse. But trying to do that while moving is taking your life in your hands, even if you can figure out how to make the desired adjustments.
Both also come with a tire pressure monitoring system, which is a great idea. However, both cars made a trip to Global Imports in Marietta when the warning light came on. With the M5, it was just a low pressure setting, possibly because of the cold weather overnight. The 650i spent a day in the shop having the run flat tire replaced because of a nail at a cost of $546 to BMW.
Both BMWs are wonderful machines but overly complicated in their operation and styling.
The M5 boasts a mighty 5.0-liter V10 engine that can be mated to a seven-speed auto/manual or true manual. Go for the true manual because the auto/manual is terrible. Whenever you shift, the car accelerates, then pauses as it shifts. Your head goes back and forth with every shift. You can adjust the pause, but never fully eliminate it.
The M5 comes with BMWs MDrive system that allows the driver to adjust the car in any number of 279 configurations. We got through about seven.
The 650i is a much easier driving car with a 4.8-liter V8 with 360 horsepower and a seven-speed auto/manual transmission that doesn’t come with the built-in lurches. The paddle shifters on the steering wheel change gears seamlessly.
Where the 650i suffers, according to some, is with the exterior styling. Available only either as a two-door coupe or convertible, the 650i is sleek and lean but the huge rear end just ruins the effect. In contrast, the Jaguar XK flows from stem to stern.
Both cars are immaculately put together with elegant interiors. The 650i doesn’t have a touch of wood, real or otherwise. Instead, touches of brushed chrome nicely compliment the two-tone interior.
The 650i is a four-seater, but the rear seating area is very tight and difficult to get into or out of. The M5, as a four-door, offers more room and easier access.
For some, either BMW would be a dream machine. But for $90,000, we’d prefer something a little less complicated.

excerpt From: http://thecarcouple.com/node/12