15s look fine on a Mk3, all things are relative...
17s will look better but destroy the ride, lessen the handling and significantly add to the unsprung weight. Also, someone more technically minded than me

will give you the correct term for the fact that larger wheels have more weight further from the hub, so due to centrifugal force will not spin (therefore accelerate) as quickly. Also, it increases the gearing, again not great for your acceleration!
Forget the wheels, is posing or driving more important?