Almost everything dude...
They're the same car.
The Golf R front brakes are massively larger than the GTI brakes. The calipers are completely different to account for that large change. The rear brakes are also significantly larger.
Of course the seats would be swapable. In fact, the standard cloth and the leather seats are the identical design, only differences in materials and logos.
The engines are different, and so you need to be sure that anything which is designed for one engine but not the other is identified. Obviously there is also the AWD vs FWD and considerations for that.
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