I also considered the mx4800, non turbo, but I am glad I went with the 5200 with turbo. Turbo saves lil fuel. The mx5200 is better on fuel per hour then my 30hp b series kubotas and I think the turbo has a little to do with that. The only thing that bugs me about a turbo is the couple minute cool down time I have to wait before I shut engine off. My non turbo tractors I use to own I would just throttle down, count to 10 and shut off, with turbo after working engine, book says to let engine idle for 2 to 3 minutes for cool down as to not cook the turbo bearings.
Also, the 3ph on the mx4800 is different, with turn buckles for tightening instead of the pin positioning for slide and tightening adjustment. Owing both types of 3ph, I am loving the 3ph style of the mx5200.
As for ag tires or r4s tires, you must decide which is the single MOST important factor for your needs, width/stability/less lawn damage, OR traction. The ags will be better for traction in wet areas and in the woods.
I chose the r4s, because I priortized the width/stability/less lawn damage bases on my business needs and that I could easily compensate for lack of traction a little bit by using tire chains, but I could not find a easy way to compensate for the ag tire short commings. (Search my previous thread in the tire section "r1 ag or r4 industrial tires")