Just yesterday I had the opportunity to play with a JD 4310 and man is that one nice tractor. I only wish that I had the opportunity to actually work the tractor in some mud or out in a field, unfortunately that was not possible yesterday but I am looking forward to it in the future. I'm a long time fan of the ergonomics and design on the New Holland tractors and up until yesterday I would have argued that they are the best thought out tractors on the market. And many of them, I believe are very well thought out (the TC35 & 40 are supurb). But the 4310 is simply supurb for its size (it is smaller than the NHTC 35 or TC40). When I sat in the seat there were things that struck me as being odd, but as I worked the controls, my hands simple fell on the controls as if they were extentions of my arms. Silly little things like the "cruise control" button that was placed on the right fender instead of the dash all of a sudden became genius concept as it essentially fell under my thumb when I was adjusting the levers I could simply engage the button without having to reach for the dash. I also was impressed with about 100 other little touches that made sence and worked as they were designed, I liked the cubby holes to hold gloves, sunglasses, cups, I liked the way the controls felt, etc. It was simply a great design and it was obvious that they really took time to study comfort and useability.
One thing about tractors (of all makes and models) is that there is something for everyone out there. Low priced low featured, modest price well featured, high priced and fully featured. I believe some are better values than others. I also believe that sometimes you get what you pay for, and I admit to not knowing what the JD4310 costs, but whatever it was, I think it was worth it.
I don't like machines that make me sore when I get off of them, or worse yet make my muscles ache for a day or two later. I'm willing to pay for the little things. I understand that not everyone can, or will, and I respect their choices. But if you plan on spending much time in the seat of your tractor, make sure it fits you very well, make sure the controls seem natural. If you are uncomfortable, you won't be safe because you will constantly be distracted by something.
As for the parts being hard to come by on the Kioti brand . . . I would suggest that is bull. My Kubota dealer told me that the Kiotis are inferior in every way. I've seen a few, played on a couple. I own a Kubota and a New Holland. I have to tell you that Kioti is probably as well built and parts are probably as available as any other brand. One thing NO manufacturer has been able to match is the smooth running Kubota engine, but as for parts and quality, Kioti is probably as good as most brands. Each is different in some ways. In some areas Kioti is a bargain, in other areas it is not, but pricing is very regional for all brands from what I can see here. But whatever you pay, make sure you find a machine you really like and that fits you, that is more important than price over the 20+ year life span of a tractor.