>>40600163Dynamax is only 3 turns though, Megas are permanent and Z-Moves are a one-shot
Megas would be preferable for the Pokemon that need them like Charizard(Gigantamaxzard will never ever be as good as Mega X or Y), Z-Moves would be preferable for some Pokemon that need them, and Dynamax would be its own thing that like Z-moves any Pokemon can use.
Z-Moves are stronger, but what really makes Dynamax its own thing is the moves secondary effects.