If you are chartering with small children or older people who have trouble moving around, a catamaran may be the best choice. In general, catamarans are more stable both while moving and when they are in the bay. Also, they have more deck space, so, for example, toddlers have more room to move around and explore. Catamarans also help people who are sensitive to the sun and make it less likely that they will get seasick because they move around and don't stand still.
Again, experience is important, so when choosing a boat, you should think about both your level of skill and that of your crew. Do you really think you can drive and dock a big catamaran or 55-foot sailboat without a bow thruster? In the same way, mooring a big boat in the Mediterranean on a small dock in Greece or Croatia isn't easy without a good captain and some experienced deckhands. If you're not as experienced, going smaller and simpler also means you'll have less problems on your holiday.