Aluminum alloy 6061-T6 is the most common, and has a yield strength of 35,000 pounds per square inch (psi). The most common structural steel is ASTM A36, which has a yield strength of 36,000 psi. Therefore, for the same cross section, the aluminum has essentially the same strength as steel, and at 1/3 the weight. There are of course much stronger alloys available for both aluminum and steel. 2024-T3 aluminum is one of the strongest with a yield strength of 62,000 psi, while 300M alloy steel can be heat treated to 260,000 psi. I doubt that you will be using either of these in your backyard though.
Also, Young's modulus (modulus of elasticity) measure stiffness and is completely different from strength. For any given load, aluminum (E=10^6 psi) will flex about 2.8 times more than steel (E=28^6psi)
And finally to crackerman2525, carbon is most definitely NOT used as an alloying element in aluminum, and it is NOT what makes aluminum "aircraft grade". The density of aluminum is 0.10 #/in^3, and steel is 0.28 #/in^3.