I have been on both Crystal Serenity and Cunards Queen Mary 2 (not the QE2 referenced above).
I liked them both. I would have to say they are very similar experiences while at sea. I think the cabin service on Crystal was far better, but the Queen Mary 2 is a fabulous ship that impresses with a grandeur that Crystal doesn't really portray. Crystal is a beautiful ship, but not a grand ship.
I would say the onboard experience was great on both of them. I tended to enjoy the nightlife on Cunard more than on Crystal. Both have e cellent daytime enrichment lectures, plus movies and other activities. Also great libraries and computer learning centers.
I would say if you like one you should like the other, unless having a butler is extremely important to you. (then go with Crystal).