I have cruised (some more than once) on the Mercury, Millennium, Constellation, Solstice and Equinox and am hosting a group on the Century next month, so I have pretty broad knowledge and experience.
I must say that the S Class has really won me over after being on a Pre-Inaugural on the Solstice and a 13 night Med cruise on the Equinox. It would be my first choice without question. I would still book the M Class ships itinerary dependent, but would probably compromise the itinerary so that I could sail on the S Class.
As for the Century...I will let you know!