M5.
with the m5 be prepared to toss in a clutch as the clutches are a known weak spot on the m5 and they do happen to go out quite early. (common)
oil changes will be 150 a piece. an inspection will be $1000-1500 every 15-20k miles.
with the M5 budget for about 3,000- 3,500 a year for upkeep (tires, brakes, oil changes, inspections, clutch, arm brushing etc)