Crunch is better than both.
It has superior power to Payback, plus a nice secondary effect. Also, Mega Tyranitar runs Dragon Dance the majority of the time and won't always go last. However, if you have to choose between Payback and Giga Impact…
Payback > Giga Impact
Giga Impact doesn't offer anything new coverage-wise, is resisted by many common types, and leaves you vulnerable for a turn while recharging. While this is naturally detrimental to every Pokémon, it is especially bad for Tyranitar since he basically gives Fighting-types the free switch in. Payback works well with slow, bulky sets, but Crunch is still the superior choice by a lot.