Do you know if the wheels are 17in or 18in ? If 18in they weren't fitted at the factory. Are you sure they are not just the standard 16in Teledials?
General opinion on here appears to be: 16in for best ride, 17in best compromise of ride comfort and looks, 18in can be a terrible ride.
It would also depend greatly on what suspension was fitted. The above only applies to standard JTD springs and shocks. If it's had a decent upgrade and has been lowered then I would say 17in would be best all-round bet. 18in would be hard on your fillings!
I can't see bigger wheels having any effect on the geometry. Someone correct me if I'm wrong tho . . .
Note: I only ever drove my 156 JTD SW on 16in so I have no personal experience. Only going off previous discussions on here. I know that when I swapped out the standard suspension for Eibach Pro shocks and springs the ride was much, much better for both handling and (surprisingly) comfort.