If you are looking for a party atmosphere, not formal, plenty of "bucket of beers" by the pool, I'd go with Carnival over RCCL. RCCL is going to be a little more sedate and more family oriented, though in general the shorter 4/5 day cruises are less formal. For 20's guys, go with Carnival. However, do be aware that the cruiselines have age limits when booking so you need to check that. I believe there may be limits of 21 or 25 yrs of age. Their websites should tell you.