If you'd ask me, what gets the car moving is torque. The 640d has 630nm of torque and if I'm not mistaken the 650i has around 600nm of torque. You'd start pulling away if you were on the autobahn. I can't think of anything else if you'll were both pushing equally as hard.
2012 BMW 640i (AC Schnitzer Package)
2011 Mercedes E250
2007 Ford Mondeo
"Age is but a statistic that has no relevance to enjoyment of horsepower"