Aluminum is stronger, won't bend, can break or crack, more expensive and lighter, polished aluminum requires regular maintenance to look good.
Steel bends, is heavier and cheaper, and easy to maintain, tho' it rusts if exposed to air, i.e. scratches that aren't touched up.
Depends on what you want to do with them. If you are a serious rock crawler, go with aluminum for sure (Jerry Bransford says so..
) Other wise, go with what you like and can afford.