Which cruise to choose!?

i'm trying to book a cruise for next may (2010) and all the choices are overwhelming! there is going to be 8 of us in the 21-26 age range. can anybody give me as much insight as they can into both norwegian, royal caribbean, and carnival cruises? should we do a 5ight or 7night? i think i'm going to do a western caribbean cruise. also, should i book the cruise now, or wait till january for "wave season"? and last, how much money do cruise lines usually tack on for airfares? is it better just to book airfare to florida myself? thanks alot!