I don't think you can make a bad choice either way. The Carnival Splendor and Mariner of the Seas will be sailing the Mexican Riviera next summer.
I personally would go with Mariner of the Seas if you have a wide range of ages. There are activities for all ages and this class of ship has everything from Royal Promenade parades to rock climbing and ice shows. It also has the standard activities such as trivia, pool games and the ability to just sit and stare at the ocean. The kids club also has many activities for the younger people in your group.
I would book with a travel agent that you can develop a one on one relationship with. Even if an agent or agency has an online presence they can still be the standard brick and mortar business. You can email me for a recommendation.
Take care and enjoy,
__________________ Cruisemates Community Leader/Moderator
"There is a great difference between being well traveled and just having been to many places." ~Me