No doubt ROTA are up there for cast wheels, its why I got a set of GTRs. They're great for their money but they're not forged wheels if thats what you are after.
So cast OEM vs cast ROTA, usually ROTA will be lighter I'd imagine, as OEM will just want to keep things as cheap as possible, but vs forged, you'd expect the forged to be lighter and about the same strength.