[MrForbes]

Steel is 2.8 times as dense as aluminum, it doesn't matter what alloy. But strength DOES depend on alloy and treating, and you can find steels that are several times stronger than some types of aluminum...and you can also find aluminum alloys that are several time stronger than mild steel! so you have to be pretty careful in you comparisons, to make sure you are comparing apples to apples, and oranges to oranges.

1501 has used chrome-moly steel tubing.

If you are interested in keeping your robot from falling over, go heavy on the chassis, and save the exotic materials and super lightweight design for all the "up top stuff", such as arms, etc.