There were several threads in the last few years about those very seat choices. At least one I remember compared the standard Kubota seat to the Kubota Air Ride seat that was an option in the tractors in the L6060 size. I believe the consensus was the standard seat actually had the best suspension. The air ride bottomed. At least the one in the article did. You may want to see if you can find those threads.
Grammer makes a bolt-in air suspension seat for most everything on the market. Price is about the same, and they are excellent seats. JD either used Grammer or an exact copy for their commercial backhoe seats. Wonderful seat. If I ever decide to change to air ride on the M59 I'll go for the Grammer since they make one in the 800 dollar range that is supposed to be a bolt-up for the M59.
In the meanwhile, I put some risers under the seat of the M59 to accomodate my long legs, and incorporated some rubber bushings to work with the risers to soak up some of the shock. I considered modifying the seat springs but didn't. I weigh 190 and only bottom them on bumps.
I like a good seat and feel that it is money well spent to get a good one with decent arm rests. The best seat I'vw used is the Grammer-made seat in the JD310. It is made to sit in all day. For comfort, the Grammer is closely followed by the seat on our Vintage 530 JD that not only has adjustable springing, but also a shock absorber!
Some of the basic "too small" Japanese tractor seats do have the option to add more springs. That used to be a common mod.
rScotty