Hi clm4700, welcome to Cruise-Chat. Does the Mercury have an itinerary you like better? How far out is your cruise, as there may be a penalty for switching? Did you book through a travel agent? If you did book through a travel agent, that specializes in cruises, they can point out the differences/similarities of the two ships. Is the fare part of your consideration? My suggestions, book both
Let us know what you decide. Best wishes and once again, welcome aboard!