Neither, I would say.
I think the Mk3 shells are pretty good for their era and wheels that are any bigger than 13" or 14" will just make you torque steer.
The front suspension set up, on the other hand, is poor. If you want the car to handle go for one of the subframes that Polo Performance make and an LSD if money is no option. Or just replace some of the ageing rubber mounts for newer poly ones.
Not saying that this is gospel, but this is a far better direction to head in rather than the typical barry answer, big rims!
Cheers
Craig