I haven't been on the Spirit specifically, but I've sailed the Spirit class. My thoughts:
The Spirit class is the best of "old" Carnival. They are laid out perfectly, have tons of space and are amongst some of the most relaxed feeling ships in the fleet. To me, I'll sail Spirit class over anything Dream class and older.
Horizon is a very good representation of "new" Carnival, with tons of food and beverage options, tons of different things to do, and all put together in a huge ship package.
If space is your main concern, go Spirit. If activities are your main concern, go Horizon.