Cruises in the Mediterranean are more about the ports than about the cruise line or ship. I recommend you find the itinerary that interests you the most and then look at which cruise lines do that itinerary. This narrows things down a little bit.
As to selecting the cruise line, it depends on your lifestyle and what you expect. What is good for me may not be ideal for you. Some prefer a more active nightlife while others want a quieter more classic experience. Give us some more background and our expert members can help guide you.