The X350 is only slightly more robust than the box store Deere you get at Lowes and HomeDepot and has the same weak transmission. Google “failed Tuff Torq k46”.
FWIW, I owned an X320 20 years ago before upgrading to a used X485SE.
Let me go against the group here and say neither of your choices is especially good for hills - the ZT because of its design and the X350 because its tranny is designed for FLAT mowing. It has the same transmission as the box store Deeres. You’ll have to go to a high end X3xx or something in the...