Congratulations on your wedding and your choice of a cruise for your honeymoon. I have not been on either ship; but Navigator just looks way more impressive, especially at the same price. But I know Fantasy has a lot of fans here; and I personally prefer ships more Fantasy's size. But Navigator definitely has the WOW! factor.
As for Cozumel, that is a place I think it is more fun to just "wing it and wander", as you say.