Cruise.com is Omega World Travel, Inc. in Fairfax, VA
Cruises.com is World Travel Holdings, Inc. in Wilmington, MA

They are different companies, as are most of the other versions of cruise or cruises dot something.

If you are going to spend hundreds, or thousands of dollars, you really shouldn't base it on a company you never heard of because they were first to register a domain name.

Not saying they are not legit or anything, just that for the price of a cruise it's more than popularity and a pretty domain name.