General rule of thumb is five pto HP per foot of cutter. That being said there are different scenarios to consider. Flat or hills ? maintaining pasture ? or trying to reclaim thicker denser brush area's..I personally use a 10 ft drag hog to maintain my pasture with no problem. with my 35 HP pto .Granted I have to cut in a lower gear,but the wider cut at a slower speed versus narrower cut at a higher speed are a wash in my mind.The thicker area's grown up with brush, I drop drop to my six footer ..Your 5570 I believe has somewhere around 62 pto hp or so if I recall.
Rule of thumb say's 12 ft, It would probably run the 15. You may check the hp requirements on the hog you are looking at if they are available. I have found that each manufacturer can at times have wide variations on their HP requirements.
Regard's
Sean