When my wife goes, she likes to go together, so we have a Tandem. Unlike the tandem bicycle, she gets to sit in front! That way I can never leave her behind and she likes it that way. When my wife doesn't go, I have taken the Tandem out with a (small) weight in the bow and that also works fine. But I also have a Classic and an Adventure and usually take one of those out. Lately, it's been the Adventure.
Other issues: My wife gets wet when I paddle the Tandem, and since she has a tendency to melt, I can only pedal when she's aboard. The singles are easier to handle if you're going alone. The Outback has lots of trays and equipment-mounting spots if you like to fish. The Adventure is faster and cuts through nasty weather easier -- will take you greater distances. It also has the largest solo weight capacity. It also is designed for sailing (daggerboard well and larger rudder available), and tracks the best for paddling. The Classic and Sport are pretty manuverable in tight spots (no experience with the Outback). The Sport is the most weight-limited (185 lb.) Any of the others can also sail except the Classic.
You can always start out with the Tandem and go from there!