Bobad, Here's my 2 cents.
Redwood is very nice, doesn't warp or split, has a nice dark red color and is very resistant to rotting. It is VERY expensive. I tried to find a price for you but the company I used to get it from hasn't stocked it for 10 years. I try to avoid non-stock items when possible.
Cedar is nice, weathers well, needs sealer occasionally, readily available, looks nice but isn't very strong. Usually cedar decks are framed with treated pine and covered with cedar. If the frame was treated pine with cedar 5/4x6 decking, the joists would need to be 12"o/c instead of 16" o/c for most composits and treated pine 5/4" decking.
As for composit, it is looks nice, you can choose from a limited amount of colors, needs no sealer. Not as slippery as other materials when wet. Shouldn't crack, warp or split. It is usually framed with treated pine and covered with the composit decking. The exposed exterior joists can be covered with vinyl fascia cover. Pilot holes have to be predrilled with a countersink type bit before screwing it down. It is expensive.
approximate prices for decking
Redwood: wouldn't consider
Composit: $4.50 sq. ft.
Cedar: $1.95 sq. ft. + sealer + extra framing
Treated Pine: $1.35 sq. ft. + sealer after 1 year
There are several types of fasteners made of various materials/coatings that are approved for use with the new treated pine. If you go this route just make sure you buy the right type and you should be fine.
If you use wood, one of the biggest mistakes I often see is that the lumber is not crowned during the install. Look at the end of the decking board. When it is laid ready to be attached the rings should look like the sun is rising. The wood will shrink and the board will cup down resulting in a smoother deck/dock and water will run off more readily. Joist should also be crowned. Look down the narrow edge. The board will arch up or down slightly. Put all of the crowns up. Just some tips if you didn't already know.
