The idea of differential lock is for the added traction when you need it. ZT's tend to tear up the turf when turning. Unfortunately the only steering wheel ZT that I am aware of is made by Cub, and they have a horrible build quality. I'd stick with a tractor .
If you have the diff lock engaged on a tractor, it will tear up the turf more than a ZTR. I DO agree that's it's unfortunate that steering wheel ZTRs are only available from Cub. I got lucky, and came across a good deal on a Pro Z 100 S, which is a commercial unit. The residential Cubs, as well as Troy Bilts, Craftsmans, etc CAN be reliable mowers, but they can't be neglected or abused. They will not tolerate anything but good care. They don't have to be babied, but you won't get away with using them like a bush hog, never changing the oil, never servicing or cleaning the deck, and leaving it outside. If I had slopes like the OP has (and I think I do), and I had to choose between a durable lawn tractor, a durable lap-bar ZTR, and a steering wheel Cub ZTR, I'd go with the Cub ZTR. It's really a game changer on slopes. My Pro Z S replaced a lap-bar ZTR, and it feels WAY safer and more sure-footed on slopes. If the engine dies on my Pro Z, I won't be replacing the mower, I'll be replacing the engine.
Edit:
Cubs can be had with fabricated decks. Costs a bit more, but I'd opt for one of those over their stamped decks.