Good question. I'm interested to hear from some other more knowlegable folks. This thread might get a better response in the mechanics forum or from the rec.bicycles.tech newsgroup.
As I understand it, 531 is a proprietary brand from Reynolds. It's a similar alloy (1.5% Mn, 0.25% Mo, 0.35% C) to 4130, but the Reynolds company applies some extra heat treating to make the tubing stronger (??). I don't know if that equates to lighter, such as 631 or 953(!), but it is a step up from basic 4130. Please, someone with more info step in...
That being said, I've got a "cheap" 4130 Cross Check and I really enjoy it. Surlys may be a bit overpriced in terms of frame technology, but they're such a versitile and rugged design.