We were on deck 9 and liked that it was only one deck up to the promenade deck and 2 up to the pool deck. The level with the main dining room was 2 levels below us so it was a good and central location. Being midship is always the best for less motion but to go up you need to go all the way forward or aft for stairways or elevators.
We've sailed on Zenith twice, both times in outside cabins--once on deck 5 and the other time on deck 10. Our cabin on deck 10 was a "premium" category 3 and had a fridge and small sofa. Otherwise, the cabin on deck 5 was identical (just a chair instead of the sofa and no fridge).
Midship on a lower deck is best if you are prone to motion sickness. Personally, I'd book a lower deck because it's less expensive and the cabins are all alike anyway. Some of the cabins on deck 9 have the view obstructed by lifeboats.