They are basically the same. The Horizon entered service in 1990, the Zenith in 1992.
They are both 46000 ton ships, have the same layout, etc.
I have sailed both but was on the Horizon for the 2nd time a few months ago and it had been well maintained. From reading posts re / the Zenith, it too has been well cared for.
You wouldn't go wrong on either, keeping in mind they are not new, glitzy mega ships with all the bells and whistles some ships have today.
If all else fails, flip a coin.