If you want light and durable go with AR400 steel 1/8" more resilient than the ballistic grade AR500 so there won't be as much concern for cracking (the key is clean cut holes and edges). Aluminum is great if you want it to look nice or be stronger than factory plastic, you can also buy 3/8" UHMW plastic sheets from McMaster or your local industrial supplier which will be stronger and slipperier than aluminum. You can form the UHMW in your garage with a heat gun and easily hand drill the bolt holes, counter sink the holes and use flat head hardware or you will have problems with bolts bending.
Then there's good old mild steel, 3/16" is fine for moderate trail use and 1/4" is not quite enough for rock crawling (except under the front diff).