Tilemanct,
I obviously like the 3720 since I bought one, but each machine would have its pluses and minuses. My opinion is as follows: I think the 3720 is a great tractor. It, in essence, is a "hot rod" of a tractor. So far, I have found it very manuverable for its size, extremely comfortable and stable. It also does strong work with the loader. It easily outdoes the 4310 I traded, even in things where not expected, such as low speed loader work, etc. If you are working in tight quarters or on soft ground, it would be your better choice. Although not small, it is smaller than the 4320. With the high horsepower, you could, if you wanted, weight it down really well with fluid filled tires and wheel weights, and have a package that weighes as much as the 4320, but in a smaller package, if heavy work is your desire. The main advantage to the 4320 is that its larger physical size allows it to handle larger attachments, which is a plus if you have a lot of land to maintain. It may compress grass more when mowing, but would still cut well, and it should offer good loader performance (but there is not 400cx loader, yet). The biggest issue I hear from those using these machines is that they ride better than the mid frames. Now that the mid frames offer an air ride seat, this may be less an issue, but the wider stance and larger tires of the 4320 may give it an edge here. I also think the 4000 series would be more stable on hills or ridges because they are wider and seemingly more "low slung". With regard to engines, the PowerTech engines in the large frames have a good reputation, just not as good as the Yanmar diesels in the mid frames. Although they are excellent, the edge may go to the mid frames here. In short, my thinking was the advantage went to the mid frame 3720 in every category other than stability and the ability to use some larger attachments, which was a secondary issue to me. Unless you have specific desires to do so, I would suggest strongly considering the mid frame 3720. I am not so sure that it is not the best tractor on the market in its size (or maybe even not in its specific size). That's how good it is.
John M