I'm sure both are good ships and would be great cruises (to Bermuda, I presume). But Zenith holds a special place in our hearts, and although we like to try different ships, I think I would go with Zenith. Celebrity is a cut above, in our humble opinion.
Been on them both to Bermuda. Loved the Nordic Empress. Great food & service, but it is an older ship that does 4&5 niters during the winter, and as such, has seen a lot of wear. My fave is still Zenith/Horizon twins.
We have cruised on both and you would be fine on either one. Food a bit better on Zenith, activities a bit better on Nordic. Those are the main things I feel are not the same. Both really are great ships.
p.s. one other would be that the Nordic has balcony cabins and the Zenith does not.