Some people complain of noise/vibration when staying Aft, as it is right over the engine. Some prefer Aft because of the views as you back into Castaway (and a few of the rooms have larger verandahs in the corners of the ship)
Some report that the mid-ship is better for sea-sick folks, since its more centrally located. Some people hate midship due to a lot of foot traffic, being centrally located and all.
Some dislike the foreward of the ship's views, and find it a bit isolated from most things. Some enjoy the quieter halls, and like being near the bridge...
So, I guess what I'm trying to say is, depends on the person/family and their wants. But one thing is for certain, no matter where you are, you're going to have a wonderful time.
I stayed foreward (right under the bridge, so I could see the captain as he steered the ship into port from my port hole), and I've stayed midship, and both were great rooms. The engine noise in the aft is probably the most valid complaint, but from what I gather it's worse on the lower decks, and it only wakes the light sleepers.