Paul,
I'm no expert on knife steel but what I have been lead to believe is the stain resistance is directly related to the amount of chromium (Cr) in the steel. I have found no magic industry-wide threshold that states if you have X% Cr then the steel is stainless, otherwise it's not. With that caveat in mind you can at least make a relative comparison that gives you a sense of how stain resistant a steel is. Admiral Steel quotes ATS-34 "stainless" at 13.75% Cr, 440C "stainless" at 17% Cr, and D2 at 12% Cr. So you could say D2 is nearly as stain resistant as ATS-34, for example.
Admiral didn't have S7 on their price list. Crucible shows their S7 at 3.25% Cr.
So the long answer to your short question is that D2 is a lot more stain resistant than SR-77 if my understanding about Cr content is correct. I hope this makes some sense <img src="/ubbthreads/images/graemlins/crazy.gif" alt="" />
