The fact that so many decorated Vietnam Swift Boat combat vets - including, IIRC, men with FAR more decorations than Kerry (Wasn't there a MOH winner in there?) came forward to denounce Kerry's record has a lot of weight; they didn't just snipe from afar or post anonymously on blogs, they stepped into the light, identified themselves, and denounced Kerry.
Either Kerry is a liar - or all of them, every single one, is.
Considering the parts of Kerry's past that we can easily verify . . . the Swift Boaters were, and remain, FAR more credible.
As for McCain . . . I'm NO fan of his, but unless a whole slew of other vets come forth to denounce him as Jane Fonda's main squeeze in Vietnam or something, his service record isn't a target for attack.