out of the 4 pto driven implements I have, tiller, PHD, rotary, and rfm, that 72" rfm by far, put the least strain on the tractor, like none. If you are trying to cut taller grass?? with a rfm, you better raise it, or use a rotary cutter, The first thing that you will damage when you clog the deck is the belt in the rfm, not the tractor, pto shaft or gearbox on the rfm.
You already have the 60" rfm, it will do great,
If you can swing a deal for a 72" in the mix, great to,
It is nice having the rfm 4" plus wider per side than the tractor when mowing the lawn.
If you do step up to a 72" rfm, get the rear discharge deck, it is better for appearence, closer cutting on both sides, and cutting taller grass