A tandem duckie will carry either one person, two people, or one person with enough gear to self-support or carry your own load on a trip and still be fun on a daytrip. A single duckie will carry one person and a personal bag - period. Think about what you want it for before you buy.
If you're bargain hunting, see if you can come up with a few hundred bucks more and throw down for something good. Sevylor proves the adage "there's the right way, the wrong way, and the French way." The valves are finnicky but you can make them work if you're careful about getting them screwed in. You'll also be sitting in about two inches of water unless you weigh <100 lbs. I was with a friend with one of the new line of inexpensive Aires and the valves were pretty odd on that one to - I'm not sure how she aired it up but it looked like it needed a special attachment.
When you're shopping, look for D-rings to hook thigh straps to and how you'll be able to seat yourself so you can brace yourself rather than swim. Also consider whether you want a bladder boat, check out how the valve assembly is secured, consider the strength of the fabric, check out the bailing holes, think twice before you buy. You may be able to find a really sturdy one (I'm kicking myself for passing on a Hyside once) at a good deal when rental fleets go on sale in the fall or next spring. That's about all I know. Ask Perry (aka "Will raft 4 beer" aka "the guy who knows a breaching narwhale when he sees one") since he's just gotten a ducky and will probably have some ideas I've missed.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse