That's a very good point, and I think there's truth to that. Sirius pranking Snape into finding Remus definitely seems... less than understanding. My guess is they're more like... typical guy buds, you know? Whereas James and Sirius were more than regular close friends. Kind of like Harry and Hermione versus Harry and Ron, although I have a hard time believing Harry and Hermione would ever think the other was capable of killing, say, Ron and Neville. On the other hand, look at the mess they were at times OoTP through DH - MWPP were fighting the war way longer than that. :/

I'm sure I have entire theories as to why Sirius suspected Remus saved somewhere. :P