I haven't sailed on the Horizon, but the two Celebrity cruises I've been on were my FAVORITE cruises of all. I love the whole Celebrity experience. Haven't been on Royal Caribbean but you'll find that the experience is a little more refined and sophisticated than Carnival. Don't get me wrong, it's still fun, but there's a touch more elegance. And the food is out of this world!
You are in for a treat!
Cheers,
Michelle P.