What type of aluminum are squares and 72" straight-edge rulers made of?
Aluminum is made of aluminum - processed from rocks.  This is actually a great "How It's Made" Episode.  There is very little that really changes with most aluminum that you buy other than size and shape.  I am sure there are slight differences in strength due to tempering processes and coating but this is usually minimal.  If some aluminum is a lot "softer" than others it is probably because it is not pure aluminum.

Steel is an alloy.  There are literally thousands of different steel mixtures.  Depending on how they want the steel to turn out they add (or take out) different minerals - including aluminum sometimes.  Steel is basically a mostly iron alloy and a lot of its properties are subject to the amount of carbon.  I think if the carbon amount hits 2-3% steel becomes very rigid but also is very brittle.  A ruler would not have a high carbon % in the alloy because it would need to be very flexible (relatively).  

Steel for the most part is "stronger" than aluminum but it depends on the alloy composition and the tempering process (which has a stronger affect than it does on aluminum).  The strongest steel is much stronger than strongest aluminum.