The 4320 is a lot more machine than the 4310. It is larger by quite a bit and heavier. It will handle heavier attachments and will do more work much more quickly. Having owned both a 4310 and a 4520 (same size as 4320) and now with a 3720, I can say the difference in the 4310 and the large frame is huge. The bigger question is whether the larger frame machine fits in places that you wish for it to,and whether you have tasks that will truly allow you to use the difference in size and power to its maximum. For example, I really liked my 4520, but about 1/3 of my property was inaccessible to it due to its size, so despite the fact it was twice the machine as my 4310 was, I could not use all it could offer due to restrictions unique to my property. For me also, the ability to trailer my tractor to different places easily was a major concern. With loaded tires, a FEL and a 72" BB, my 4520 weighed approximately 7000#. With my trailer pushing 3300#, even my 3/4 ton pickup had a load. Not for pulling per say, but from braking and handling. A comparable mid frame will weigh about 2000# less set-up similarly. Hopefully this offers some food for thought, but if one had the need and the means by which to get a large frame, one should in my mind. But for me, a higher HP mid frame was a better choice.
John M