People have different reactions to how much, and where, they feel the most movement. We were midship and I felt it worse than my friends who were aft. And I felt that motion wherever I was on the ship, really. I think sometimes it's the person's individual reaction.
There are other factors to take into account. Midship was in the center of everything -- but that's just it, it was in the center of everything. The midship elevators can get ridiculously busy. And you're always going to one end of the ship or the other. For us, we'd rather be forward, next time, because forward puts us closest to the adult-only areas (pool, Satellite Falls, Cove Cafe) and the theatre. Some might like aft because it's closer to the clubs and the restaurants. It depends on what you want to be near.