If you’re shopping for a sunny Caribbean cruise, chances are ships by these lines will come up in your research. After all, both lines cater to everyone from small kids to retirees, as well as millennials and baby boomers, with a couple dozen ships that hold thousands of passengers each. Both also offer fun, lively vacations that are packed with activities and entertainment options. So how do the two lines stack up against each other? Here's our take.  AtmosphereFor decades, industry giant Carnival has delivered casual, unpretentious fun for the young and young at heart (think poolside dance parties) at an affordable — and often bargain — price. A lot has changed and the new ships are less gaudy and — dare we say it — even tasteful. The line has made efforts to become more family-oriented, with activities such as family game shows, plus new family accommodations on Carnival Vista. If you haven't tried Carnival in awhile it may be time to revisit this budget-friendly option. vs. Royal Caribbean has for years operated under the theory that bigger is better. To us, the big crowds are both a plus and a minus. There are actual parades down the indoor promenade on the larger ships, but finding a lounge chair on deck can be a challenge especially on the larger ships, where you may be one of more than 6,000 on board. These cruises are about being active and sometimes even sporty, with activities that would fit right in at any amusement park or modern fitness center.