If I was a betting man. I would bet the Chargers under Garrett will win more than the Cowboys with McCarthy.
Depends on talent acquisition and coordinator choices.
If Garrett is given what he had on defense here for most years and chooses to use his offensive philosophy, then he's not likely to win anywhere else than he was here. If he has the offense he had here plus a really good defense put together for him, then that could make the difference.
I'll say the same thing about McCarthy. Give him a top-notch defense and he can win a Super Bowl with this offense. Jury's out on whether he can improve the offense enough over what Garrett could offer to carry a so-so defense.