I'd say Radiance Class or Vision Class are going to feel like the M Class. The Quantum Class are going to feel a bit like S Class. Oasis Class are great, but there are slides and carousels, etc. However, I think it's very possible to avoid those areas and spend time in Central Park, for example. Voyager and Freedom Classes are going to be smaller versions of Oasis Class.
If my recent Millennium cruise was any indication, I'd say Royal is going to have more going on, more activities, and more music than Celebrity. I think Celebrity had better food and service was similar.