I think if the Vikings signed him, it would be as the surefire starter. I don't think there is any way his ego would allow him to sign with a team intending on a QB competition. Plus, McCarthy wasn't exactly a sure-fire future star coming into the draft last year, and is coming off a serious injury that is basically making him a rookie again.
That said, there would be no one standing in his way for a starting job on both the Giants and Steelers. The Steelers are probably a better bet for a playoff run, and have better skill position talent across the board.