My problem with the run flats on my 650 are just that they're not as sticky as I'd like. Has nothing to do with their being run flats, per se.
On topic, when I was looking at the car originally the 640 was a bit of a no-brainer from a price and efficiency perspective. Really, the extra horsepower is fun, but not really truly needed. However, the 6-speed wasonly available in the 650, so that quickly became the no-brainer choice. But that is no longer a factor, so it comes back to your own individual desires and requirements.
Both cars are beautiful. Both cars have a lot of get up and go. Both cars have gorgeous interiors. Your call.