If all you want is to go fast, look for something with ridges or raised runners on the bottom so it will have less snow contact. However you may find it hard to steer - it will pretty much just go straight down the hill unless you lean wildly, which may cause you to fall over.
The discs are more controllable (in a wild crazy omg I'm flying down a hill kind of way) and perhaps more fun since what you do affects your ride. Ditto the "crazy carpets".
Do try lying face down on your ride of choice (ala skeleton) for both a faster feel and more control. If your mother isn't watching, work on starting out by putting the sled on the ground, stepping back from it a few paces, then running toward it and launching yourself onto it face first to start your run.
The big wooden things with the curl at the front (toboggans, though some people use the word for any kind of sled) are best for piling 5 or 6 people onto and then all falling off in a tangled heap when you try to turn a corner of any kind. While this is fun, it's offset by the hassle of dragging such a heavy thing back up the hill.
$100 or more
The multipart constructions with seats, turnable skis, etc are similarly a pain to carry back up. I haven't tried the foam boards yet, but they look expensive and I'm not sure what the benefit is.
My suggestion: go to a Canadian Tire (or your local equivalent) and buy a selection of cheap lightweight plastic things: a disc, a rectangle that looks like two kids could sit in it, a carpet, etc (and if there are several of you then get several styles of rectangle) - spend about $30-$40 per person and then throw all that at a hill and find out what you like. If you're on a public hill, you can always give your "rejected" sleds to random other hill-visitors, who often show up in a group that has more people than sleds.