I was considering the X320 when I was in the market for a tractor late last fall. As others have said, the K46 tranny in the X300 is to be avoided. Most people on this forum say that X320 is the absolute minimum tractor to buy due to the upgraded tranny over the X300.
Having said that, I purchased a used 2003 GT245 with 500 hours on it for $2100 (including a JD towable fertilizer spreader and 10P dump cart). No sales tax, since it was purchased from a private individual rather than a dealer.
So I saved considerable $$ when compared to the price of the X320 (or even the X300), and got a tractor with an even beefier K71 tranny vs. what the X320 provides. In addition, my GT245 came with the 54C 54" deck, which I believe is a beefier deck than which comes with the X320. Furthermore, the GT245 travels faster over the ground than the X320. Don't underestimate the time savings that a wider deck and faster ground speed will provide.
I don't recall the engine differences between the two. The GT245 comes with a Kawasaki V-twin 20 hp engine. This thing is a mule, and easily swings the three blades in the 54C deck, even when mowing tall, thick grass.
Just some food for thought....
Steve