As I understand it, 650 and 635 both suffer from having the worst beam (largest size and divergence). 640 and 642 I have heard people say are the same and companies use the number interchangibly. But basically 640 has the best characteristics for the reds. It's also the most expensive. 638 and 637 obviously have some differences because even companies that carry both in their product offerings charge more for 637 and I believe it has the better beam profile.
Then again... it's red so, who the heck knows.