If I were buying a ZTR mower, I would spend the extra couple thousand and get at least a low end commercial grade mower. They have better hydrostatic drive units, better engines and usually heavier components. Last summer I bought a Ferris IS 700 with the Kawasaki 691FS engine. It is one grade below the commercial (FX) engine but much better than homeowner models. I am really happy with my purchase and the coil spring/shock 4 wheel suspension really smooths out the bumps. It listed for $5800 so about 30% more than the homeowner versions that you are pricing, but I think it is much more mower.
Check out the models that you are considering and just see if the hydro pumps have oil drain and change capability (look at the owners manual for service requirement on the HST, if none then it is throwaway design). The service requirement is usually a sign that they have something other than throw away transaxles. Also look under the seat and see if they have cooling fans on the HST drive motors, another indication of heavy duty design. I can step up on my FERRIS mower deck to mount the mower and it does not move unlike my homeowner grade Craftsman that kicks down to the anti-scalp wheels if stepped on. MY Ferris weighs a hefty 850 pounds compared to the Craftsman 400 pounds or so (easily picked up and put on my trailer by two young men). Check them out before making your final decision. Simplicity is the same as Ferris as Ferris is a subsidiary of Simplicity.