It all depends on what your needs are. The X300 and X320 share the same frame and controls, but the engine (17 to 22), transmission (K46 to K58) and mower deck (42" to 48") are all larger on the X320. How much area are you mowing? Do you have hills? Lots of obstacles to mow around? Will you be pulling a heavy cart on a consistent basis, or using the tractor for snow removal?
my input:
I just got the 304 literally 5 days ago after years of research(or procrastination on my part) It has the kawisacki 17 hp. I am telling you that the 17 kawi like my 23 year old j.d.180 with a kawi is awesome. My property is all hills and obstacles. I chose the 304 for the 4 wheel steer too, and i am not disappointed, at all.
The machine is solid, and what a great cut too. I looked at the other x series models but the 304 at 3200 just made too much sense.
X304 won't work for me as I need a 38" deck to get through gates, thinking hard about X320 for better tranny. Enjoy your new tractor! Good luck and have a nice day.