I would strongly urge you to continue to gather as much information about this all-important voyage/vacation of yours. If you do not know this one basic difference between the itineraries of the Grand Princess and the Brilliance of the Seas, then you have not gotten enough information from your travel agent or husband or yourself.
The Brilliance of the Seas travels round trip from Barcelona to Barcelona. The Grand Princess travels one-way between different ports of embarkation and disembarkation---Barcelona, Rome, Venice---depending on the itinerary.
If you are going to bother to "do the Med", please, for your own enjoyment, find out a bit about the itineraries.