The 3600 series has a little more fit & finish look to it & more comfort stuff I would say. It also has an available Power Shuttle Transmission. Comes with Electro-Hydraulic push button engagement of the 4wd, Diff Lock & PTO. Some comfort stuff not available on the 500 series. It also has the SISU engine, a little quiter than the perkins in the 500 series. The weights are pretty close to the same on each.
The 500 series is a straight forward "work" tractor, the kind you could put a 8 year old or 80 year old on and they could figure it out without messing anything up. Some worry about the electronics of the newer style stuff. I don't know. I can't get into the computer of my truck either, I still bought it.