Raycast collision vs unity collision system ( 2D Android game).

It's difficult to say which is more performant since it depends on how well-written your home-grown collision system would be based on raycasting. However, I can say that Unity's built-in collision system is likely performant enough for anything you'd consider doing with it in a 2D game. With that in mind, I'd certainly use the built-in collision system.

I'd guess that working to keep your own code as performant as possible is a better place to concentrate your efforts.