I don't think that there is a definite answer of "better"... everything has a tradeoff (weight, strength, price). In any case, teams have used materials other than aluminum before. See this thread:
http://www.chiefdelphi.com/forums/sh...ad.php?t=51333
I have also seen some very beautiful wood 'bots.
http://www.chiefdelphi.com/media/photos/20312
(Then again, I've also seen a few very sketchy wood 'bots.) In any case, the kitbot frame is spectacular, and if you find yourself limited in time, money, expertise, or equipment, it's almost certainly your best choice. Doing a few welds to reinforce it are always a good idea.