first off i will point out that all stander mowers are zero turn.
we should change the words we use when talking about mowers. just saying a ztr does NOT mean a sit down mower.
hydro walkbehinds are zero turn, sit down mowers are zero turn and standers are zero turn.
so if the questions is why pick a sit down over a stander or vise versa i can answer that.
i've used both and can say the stander is much more convienent and i feel saves a small amount of time on each property.
i can get off the stander much faster than the sit down when i need to move an object, empty a grass catcher, etc. with the sit down i have to engage the parking break for the engine to keep running if i get off the seat. the stander i can just hop off. plus to hop off the stander is more quicker than "climbing" off the sit down.
a stander does take up less space on a trailer.
cost has never been an issue with me. i purchase what i need no matter what the cost. so the fact that they are in the same price range as a sit down doesn't phase me in the least.
so why pick a sit down? well i'm not sure i could answer that. the last one i had for a week when my stander was in the shop made me want to have my next mower purchase be a sit down because it was so nice sitting down for a change. but i hear all the problems with your back people say they have by sitting all day plus again it took more time to get off the sit down when i had to move an object or empty the grass catcher. also i use the rops and i came across a few properties with low hanging limbs the sit down would hit or i would have to mow around, causing more trimming around the tree.
as for handling hills. the stander does an excellent job on the small hills i have but so did the sit down.