Big, comfortable, faster than Songserm and Seatran, Lomprayah Catamaran is the best choice for people who are pressed for time and want to avoid the seasick experience. If you have extra baht to burn, Lomprayah is your best choice. Otherwise, go for Songserm, my personal choice for a rather comfortable budget travel.
Longprayah is like the big brother in this ferry industry. They get a head start by offering passangers with prime time ferry schedules. They even have their own airline operating transfers to Bangkok.
You might be interested in travel route from Andaman Coast to Gulf of Thailand and Bangkok.