Steel if you want them to last. Aluminum is weak if you're as clueless as you seem to be. No offense should be taken from that, either.
well im not clueless im just trying to find the best method as in i haven't looked very deeply into making bars....
If you don't know what you're doing, I wouldn't suggest making bars. You can go ahead and learn about material selection, welding, design, and whatnot but if you have to ask whether (chromoloy) steel is stronger than (most) aluminum, you're not ready yet.