To create a normal 2D platform game, which one is better?
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There is no collision transformation with z elevation, but there is with parallax, although you still have to figure out offsets. There are expressions for parallax to make it somewhat easier.
There is no "z" scale for parallax.
Parallax is limited to individual layers. You can't add layers at runtime.
You can have unlimited z elevation.
You should not mix z elevation, and parallax. I mean you can, but echh.