Well, the stainless is thicker, and probably stronger - If I ever buy a Tomcat, that is the version I would buy. WHY is one smaller than the other, though. I do not know.
Beretta had a lot of problems w/ the initial batch of Tomcats, but worked the problems out. If the thicker gun was actually necessary, however, it seems like BOTH the blue and stainless versions would be the same size...