Here are some general recommendations. I'm not a world champion paddler, so others feel free to chime in with alternative points of view!
Winged blade paddles are more effective when you are moving at an high average rate of speed. Great for flat, open water like inland rivers, lakes, and the ocean. They excel at "gripping" the water when you are moving fast, enabling you to keep moving with less effort. I find they can be cumbersome in narrow waterways, especially when you are required to turn frequently.
Traditional flat bladed paddles are best for other paddling conditions, such as navigating the narrow and often debris-filled waterways you'll typically encounter in small rivers/creeks/swamps and when your strokes are often varied (lots of draw strokes, bracing against logs, etc.)
Personally use both types. If I had to just have one, it would be a flat blade.
Lighter is better! Get a carbon shaft if possible. If you can afford it, consider a carbon blade as well.
How many pieces? Kayak paddles come in a variety, from one (I don't recommend it!) to 5. The cost goes up as the number of pieces increases due to additional workmanship and fittings. Weight also goes up slightly as well. For a 3-8 hour race a 2-piece paddle should be fine. You typically don't have to carry your paddle for much (if any) of a short race like this. For longer races, a 3-piece paddle fits somewhat into a backpack, making it portable for extended carrying. However nothing beats a 4 or 5 piece paddle for easy of stuffing in your pack for a long (multi-day) race. I use a 3-piece carbon shaft/plastic blade paddle for most races under 12 hours, and use a full carbon 5-piece paddle for longer races.
I hope that helps!
~Dash
Team Florida Extreme
