Eric, Kuki is right on the money about the Spirit. We have sailed her twice and from the standpoint of a family of four you would be wise to either consider two staterooms or opt for the Miracle, which offers larger and much more variety in stateroom availability.
I actually like the Spirit, it has lots of eating venues, terrific kid's facilities. It is unique and has a terrific crew. But it has one HUGE weakness, it has a limited number of quad staterooms and most of those are extremely tiny inside cabins. If I recall there were also a handful of porthole cabins, but not many, and they too were tiny. If you can get your hands on a deck plan, there are a couple of larger inside quads on some of the lower decks, if they haven't been booked yet. For the past year the Spirit has been experiencing engine problems and has operated at reduced speeds causing lots of itinerary changes. That should be fixed though when it goes into drydock next month. The price is usually very attractive on the Spirit. We did a 14 day repo on her this Spring and had a ball, for four of us it was about $2400, sometimes on her New York Spring Break sailings they run a "kids free" promo.
I do like the ship, we've had a lot of good times on her, but the cabins are awfully small. and the Miracle's cabin will seem almost palacial in comparison.