I've been running a Ferris IS3200z with a 72" deck for several years. $$$, but much better build quality and materials than some of the "other" popular ZTRs running around. No problems yet. I did put a set of Maxxis Viper ATV tires on the back though. I hate turf tires.
About ten years ago I put Ag style bar tread tires on my 1000Z to help on the hills here.
My son and I swapped machines for a couple of seasons, so I've put the suspension on his IS2100 to the test.
I like the old rigid frame design if the property is flat and relatively smooth, it's just solid /simple. From what I've seen most of the ZT's on the market are made that way.
There is no doubt in my mind that besides the better ride, the IS2100 climbs hills, especially at an angle, way better than the rigid frame. It keeps traction (even w/ turf tires) over bumps and ruts better than my rigid frame. The down side of the IS is that it's so much more complex, with more moving parts that could go out of alignment/wear out. That said, we have had no problems with the IS machine and it's been run on some pretty rough hilly ground since we bought it in 2016.
After running both of our mowers my friend/neighbor went out and bought a ISX800z and loves it. It rides even better than the IS2100. I think that is mostly due to it's real cushy suspension seat.
If I live long enough for the economy to somewhat return to normal we'll see what's available in 60 inch mowers. I had been giving some thought to going to a B or BX but my trimming work would really increase from the ZT.
Right now, just looking at specs, I'm interested in the Ferris ISX3300. I like the larger tire size, (again because of my rough property), and the larger engine.
WoodDuck, which engine did you choose and why? Any regrets in that department?
My apologies for the long winded response.