I had a very basic 4200, gear trans, with a 72" mmm. Reached a point where I no longer needed the mower, but wanted a loader.
Went looking for a used similar sized tractor w/loader. Found several candidates, but the 3520 clearly presented the best condition/value combination and also had more power and features than anything else in the group.
The 4200 is the only similar siized model that I have any acutal experience with. At light loads the 3520 is louder, but not objectionable to me. The strange thing is that under moderate to heavy loads the 3520 is extremely smooth and quite. I have been told that is because of the turbo, but I don't really know. It was actually quite errie the first few times I experienced it. With a brushhog mower just cutting grass, as you hit that certain load all the sound seems to move from in front of you to behind you because suddenly you only hear the mower, not the engine. I am very happy with the 3520 but think I would have been equally happy with a 3320. I suspect there is little real world performance difference unless you need the maximum pto hp.