I personally chose aluminum. I don't have much experience with fiberglass and it seem to be fairly expensive.
My woodworking skills are not good enough to want to show them off

so I wouldn't do a wood with clear finish (varnish or fiberglass). Painting over wood is cheaper, but requires a lot of sanding and filling to get a nice finish, and is not as durable as fiberglass (requires a lot of maintence).
Aluminum is fairly easy to work with, and doesn't require any specaial tools. It has the "retro" look that I am after. Should last a long time, unless I don't get a seam or edge sealed properly. Allows me to use cheaper wood underneath, since it will be covered. It is also availible pre-finished in several different colors, if you don't like the "mill" finish.
One of the dis-advantages I have found with aluminum, is that it can be hard to find in widths over 48". I did find some 60" wide stuff, which will work well for my tear (59" wide overall), but I did a lot of calling around to find it. There is some 103" wide aluminum availible from tractor trailer repair places, but I worry that it may be too soft, and there would be a lot of waste. I know that several people on this forum have mentioned using it, but I haven't heard how it holds up yet.
Where in Middle Tenn are you located? My folks live in Hendersonville.