Sorry to chime in late, I bought an x-500 a month ago, it is a great machine I have 2 acres of rough field with x-mas trees, I agonized over spending $5000 hard earned dollars on a lawn mower. I looked at the x320 seriously but the diif lock of the x500 is what really sold me. When I took delivery of it the first thing I cut was the roughest field with knee high weeds and random grass, my first impression is wow this thing is great, it doesn't strain to mow under these conditions, also the roughness of the field isn't a strain on it, I haven't needed the diff lock until I mowed along the wet drainage ditch and then the wheels lock together and it just goes. I haven't had it long but I'm glad I spent the extra money for it. I can see getting many years of use out of it, with the 48" deck I paid just under 5000 cash.