I think I would like to stick with jd, simply for the restoration aspect. Not that they are any better than others, but I've always wanted a big green tractor..
A 4320 was sold in higher numbers and shares most of the 4020 parts making it an easier restoration project. I would not rule out a 5020 either if you want a fel on the tractor as it has a much heavier built front end. My 4020 was also a good tractor but the front end was a little weak compared to the new models. Fels were not nearly as popular then as they are now.