Interesting question. McCain might have prosecuted the war in Iraq better, especially the aftermath of Saddam's ouster; but would he have invaded Iraq in the first place? I'd bet no. I realize he was very supportive of the Bush policy, but that doesn't necessarily mean it's the policy he'd have made if he'd been president. He'd surely have ratcheted up the pressure on Saddam, but I think he'd have been more open to persuasion by the State Department, the Defense Department and the Europeans not to do pull the trigger. After all, the major personnel throughout a McCain administration would have been importantly different, and I doubt they would have been as inclined toward the view that Saddam had to be removed. I'm not trying to make a judgment about the comparative wisdom here — just hazarding a guess on what might have been.
That sounds totally wrong to me. My impression of McCain is that though he was a believer in restraint back in the 1980s, that by 2000 he was the neocon in the race. There was a reason, after all, why Bill Kristol and so forth were supporting him and it wasn't Kristol's commitment to campaign finance reform. Indeed, my recollection is that back during the period between 9/11 and when the Bush administration began its formal push for the Iraq AUMF that McCain was, along with Joe Lieberman one of the leading legislative proponents of regime change.
But McCarthy knows a lot more about the world of conservative national security thinkers than I do. If there's any evidence out there that McCain might not be the dyed-in-the-wool hawk he appears to be, I'd be interested to know it. It might make his seeming comeback in New Hampshire look less frightening.