I'm ordering both of them in the next couple of weeks. I'm currently arm wrestling with my wallet and can't break it down. I'm thinking of shouting out 'hey, check out that gangster roll sitting on the table', and beat when he looks over.
'Heh, and you'd think i'd fall for that?"
'Shut up wallet, you're making us look weird'.
Sorry about that. As I said, i'll be ordering them pretty soon and will post an update. In any case my impressions without having listened to either is that the 650s are a better all-around pair, while the 501s will shine for specific styles/recordings; mainly those focused on midrange sounds like acoustic music.
If you do a search for 501s you can find a lot of comparissons. The main complaint is it's weak bass, though some 501 lovers state that with the a good amp, they show tight and accurate bass, even if it isn't insanely deep.
The only experienced I've had with 501s is their comfort, which is superb (tried them on at a shop). The senns look very comfy too, once the clamp is gone, though 501s look like they keep cooler (temperature wise).