Let me start by saying both products are amazing. What it boils down to is preference. The biggest difference is the Pro Plus 36 keyboard mouse deck is on a lower level which I prefer (see images). My husband prefers to be at the same level when he stands up. The Pro Plus 36 when fully extended is about one inch taller and comes out about 8 inches farther with the keyboard tray. We comfortably sit both desks on a 23.5” (D) desk with only about a ½ inch sitting over.
I really enjoy the Pro Plus 36 but almost went with the Pro 36 because I thought I was going to have a lot less desk space. I end up being very wrong. At home I have my Surface Pro 3, my 29 inch monitor very comfortably on the desk (see images). I have plenty of room to put more items on the desk such as ipad, folders, paperwork, and can easily fit my Moleskin planner on the keyboard tray. I also have the Pro Plus 36 at work and fit two large monitors comfortably.
My husband and I work on the computer 10+ hours a day and the desks have tremendously helped our backs and our energy levels. The desks are sturdy but I wouldn’t recommend any heavy leaning while the desks are extended.