Is Chesto Berry a better option on Snorlax than it's Z-Move?
I'd personally go with Chesto-Resto. The reason being that while Snorlium-Z gives you a very powerful attack, it also means you'll have to be using Giga Impact, which is not a very good Move competitively. You can also still use Rest effectively without a Chesto Berry, where as with Snorlium-Z you'll basically be working with only 3 Moves after your Z-Move is used.
I would go with snorlium z because of its pure damage capability. (I see what you were getting at with the chesto)