I am a small operator with a 36" Exmark Viking, w/b, with BullRider sulky. In may I bought a John Deere 717A, 48" ZTR.
The production difference is not as I had hoped. The difference depends highly on the property. Some are about the same, some are faster, others are slower. Where the property is good terrain (e.g. without any difficult slopes), and there are no difficult places to mow (e.g. narrow necks, pennisulas), the ZTR is about 25% faster. But, out of 40 accounts, I only have 2 or 3 of those. The ones that are slower require using the hand mower to mow places the ZTR will not go, but the w/b will go. If I have the ZTR to mow the more wide open areas of a property, but also have some terrain that it will not mow, requiring use of the hand mower, then any advantage of the ZTR is lost. The moment the hand mower comes off the trailer to finish the missed areas, the advantage of the ZTR is gone, even though it is 12" wider than the w/b.
Mowing speed is a bit faster on the ZTR for a few properties, but about the same for other properties that are rough.
Also, another important consideration is obstacle patterns. The w/b is quicker to navigate around trees, playsets, drain grates, ect. The ZTR seems like a tank after using the w/b, and the 717A is a small ZTR.
I only use my ZTR about half the time. The w/b is used the other half. For me, the ZTR is OK, but clearly not the panacea that is so often portrayed here on LS. Yes, the ZTR is easier on my body in most places (harder on my body on rough ground), but the time savings isn't much. I was hoping to add more customers after getting the ZTR, but that was not the case. I still have the same customer base as with the smaller deck w/b mower.
Having said that, the John Deere does a much better job of mowing than the Exmark. So, aside from the productivity side, the ZTR is used as often as possible because the mowing results are much better.