I haven't been on either but I've been on Spirit, which is the sister to them. Whichever you choose, you'll have an awesome time. Personally, I would choose based on dates and itinerary. But if it came down to the ship, I'd say Miracle because it's newer. Also, Legend has Urns everywhere as part of the decor and they seem kind of ugly.
Holiday Baja Mexico 8/98
Disney Wonder Bahamas 8/00
Ecstasy Baja Mexico 8/02
Carnival Spirit Alaska 7/03
Ecstasy Baja Mexico 8/03
Adventure of the Seas Southern Caribbean 12/04
Carnival Pride Mexican Riviera 8/05