Remember, Snape was following the trio to the Whomping Willow. So he most likely heard the story of how Sirius was framed, and when he surprised them with his presence he was not a bit torn about hearing the news. So I don't think Snape would have saved him at all. In fact, I think that Snape would still have, if he had had his way, marched Sirius up to the gallows himself. Snape can hold a grudge, and even if Sirius is innocent, a lot of bad blood has grown between them. Snape can't shrug that off just because Sirius is innocent about one thing. Because he's not innocent of so many other things, in Snape's mind.