I think Carwin takes it.
Nelson showed a lot of toughness last time out, which makes it possible that he can take advantage of Carwin's stamina, but SC is bigger, stronger and has better wrestling and stand-up. I think he'll control the pace of the fight and where it takes place and will finish Nelson in the 2nd round.
Also, Carwin came close to beating the champ, whereas Nelson didn't really have any good moments against JDS. So I'd say that style and recent form favour Carwin.
Some guys are just cursed with terrible stamina, and if that's the case then he's in trouble, but I think Carwin will improve conditioning, either by changing his training, his mentality or by using less steroids (or maybe more steroids, I dunno).