Number of people on board
Chartering as a couple is very different from chartering as a group. Two people need less space and less equipment, but they might not be able to handle a bigger rig or the large main of a catamaran in a storm. In the same way, a 34-foot monohull might be just right for four people or less. Larger groups, on the other hand, will need space to spread out, as well as bigger water tanks and battery banks to power lots of lights and all sorts of different electronic devices. In this case, the best choice would be a monohull boat longer than 50 feet or a catamaran longer than 45 feet.
Also, think about how big of a boat you can control, reef, and dock, especially when the weather isn't so sunny.