I don't think wheelbase is going to be significant because we are not talking about a major difference.
Not sure about turf tires but R-4s are much more rigid than R-1s.
Brute size is the most significant factor in ride. We have 3 loader tractors. When gathering large round bales, the CUT is a back breaker going across a field, especially unloaded. Our John Deere 6230 utility is much better but the Kubota M135GX beats the JD. Weight, and the larger tractors have R-1 radials. On both of them sidewall flex is visible. The R-4s seem as rigid as Fred Flintstone's vehicles.
Air ride seats are worth the money especially if you have back problems. Not only are they better cushioning, they are easy to adjust for height so you can vary positions on long days.
As for brands, it's worth trying but if you look at identical size class CUTs and think about the physics, there isn't much that will make a difference. It comes down to seating position and what feels good. Look for things that will strike your knees because there will always be dome bounce and tractors seem to all have something you keep banging your knee against.