OkieG, my Ford has AG tires. The rears are the ones that were on it when I bought it around '93. At that time the tractor was 10 years old, and they may be the originals.
Front tires are another story. I replaced the fronts not long after getting the tractor, and the tread shows considerable wear now. I normally do a few miles of blactop travel weekly during the mowing season. No signs of cracking front or rear so far, and it's stored outside. I only use 4WD when needed, and not on blacktop except for plowing snow.
Went w/ R4's when I got the Kubota last Spring. Partly because of the road travel, and I also thought they'd be better with the FEL. So far they've been great - dirt, mud, dry and wet grass. The Kubota with R4's has done better than the Ford with AGs. Both are HST's.
After using the Ford for so many years on the same land, I am "programmed" to anticipate the tractor's feel when I get to certain locations. The Kubota doesn't slide or spin nearly as much as the Ford does. I use a RFM, and there are places where I back up a grade, and have to either lift the deck (to get weight on the rears for traction) or flip to 4WD with the Ford. In many of those spots, the Kubota simply backs up without having to do anything other than hit reverse. There are some places where it's efficient (and a bit unnerving) to mow side-hill. With the Ford, the technique includes putting the left cheek on the left fender, tractor in 4WD, and ride it out kinda "doggy" along the hill as the rear slides downhill slightly and the fronts get traction and try to pull the tractor uphill. With the Kubota, it's 4wd, stay in the seat and drive straight.
Did I mention I was happy with the R4's? Sure hope to try them in the snow a lot this Winter.