Well they both suck and I would highly debate one outplayed the other logistically. Zappe got lucky on a few of his balls not getting intercepted in his first few games, but we all saw the problems and saw him crash and burn at the end.
Zappe statistically certainly didn't. He had a lower completion percentage, lower passer rating, less yards per game, threw only 3 less INT's despite starting far less, for all the claims of him being better in the pocket he took more sacks and lost more yards on sacks.
The real issue here is that Zappe's biggest supporters basically made the argument against him being a back up. Whenever he came in as back up after Mac and played bad they argued that he didn't get first team reps and it wasn't fair to judge him because he couldn't suceed. Yet that is precisely what a back up is meant to do. So they in turn made the case that he should not be trusted as a back up.