A few thoughts,
Scag Cheetahs have an adjustable suspension platform that includes both the seat and the foot rest area.
A Turf Tiger gives a better ride as it is a heavier machine than a Cheetah and Tiger Cat II.
Tiger Cats and Turf Tigers have separate pumps and wheels motors that drive the machines whereas the Cheetah has integral hydrostatic drives. This is not objectionable, it does bring different steering and control characteristics between the two different drive systems.
Almost all ZTRs have a rigid suspension system, exceptions are Ferris and possibly other machines produced under the Briggs & Stratton lines.
Generally, larger diameter and wider tires, both front and rear, give a better ride.
Having back problems, you may want to check out some mid to high level garden tractors.
I replaced the standard seat on my 2008 Tiger Cub with an adjustable suspension seat similar to the seats found on Tiger Cats and Turf Tigers. The adjustable lumbar support was the best thing in the new seat.
Bear in mind that with any machine, particularly a ZTR, as your skill of operation level improves, you will operate the machine at a faster ground speed. This in itself will decrease the operator comfort level.
When the Cheetah first came out, there were operators that loved them and operators that hated them, I'm not sure why as these operators didn't specifically state why.
As Scags are very popular in my area, I do see more commercial operators using Cheetahs, most with collection systems.
I suspect the reason for the increase in Cheetahs is that the price of a new Turf Tiger II with a collection system is close to $15,000.
Although Scag has an attractive financing program, the payments are still due monthly during the off season, just like boat and RV payments, none of which are able to plow snow!!!
Also bear in mind that Scag doesn't extend the warranty passed the original buyer.
The JD dealer that has the Cheetah for sale may give you a 30 day or longer warranty which they have included in the sale price of the preowned machine.