Hi, I am currently booked on the Solstice for the 30th of this month. I have booked a cabin close to the front.
I can say that I have sailed on several Celebrity ships all over the globe and have yet to experience any real problem with motion. On the Millenium class ships I usually book conceierge in the back of the hsip and still I have no problem with motion.
This may come into play if you hit rough seas like I did on the Horizon, but even then it was not bad.
I would say pick a cabin you really want and don't worry about the motion of the ocean, it really won't feel it much at all.