The M Series are the fancier models, with all the bells and whistles. They have electric switches that do turn on and off the pto, 4wd and the slide levers for the Hydraulics are much easier to deal with and they have a much heavier 3 point hitch with the piston type and not a rock shaft type setup. Basically they are a baby 6000 series. Very nice machine. Also, very spendy as well.
The 5083E Limited is a tractor that is trying to bridge the gap between that tractor and the very plain and stripped 5045 and 55 tractors that are open station tractors.
It gives you a lot of nice features that you won't find in tractors in that price range, like the e pto and a hydraulic shuttle, and a cab. The levers are old school manual types levers that don't work as smoothly, and are a bit more cumbersome to use. Doesn't weigh as much as the M series. But, doesn't cost as much either. It is JD value priced cab tractor.
I just traded my 3520 for one last week. I don't have it yet as they are putting the loader and extra hydaulic outlets on it this week. I hope it will be closer to the size I need for the farm work. I just couldn't get my arms around the price of the M series. It was a stretch to get to the 5083E. I am hoping it will work out okay.