Carnival vs. Royal Caribbean. Europe vs. Caribbean.?

My fiance and I won a 5 day cruise on either Carnival or Royal Caribbean. Were not sure which cruise line to pick and whether of not we should go to Europe or the Caribbean. We will both be 20 on our honeymoon. (our cruise broker ured us that it is not a problem as long as we have our marriage certificate.) but we are not party people at all so we would prefer the lesser of the party ships. Which cruise line is better? What are things to keep in mind (other than obviously we have to pay for airfare taxes tips ect)? What kind of excursions off the ships/fun things on the ships does your preferred cruise line provide? And should we exten our cruise to seven days?