Steel elastic modulus is nearly 3x that of aluminum:
E[sub]st[/sub] = 29-30 Msi
E[sub]Al[/sub] = 10-10.5 Msi

Depends on the alloy. But generally, aluminum can make for lighter-weight structures that take bending and torsion (any non-membrane type loads), due to it's sheer bulk. It's 1/3 the density of steel, so you can make beefier sections that more than make up for being weaker by having much higher moments of area/inertia.

I just cut the weight of a critical structural component for a new vehicle design in half by making it a thicker-walled, bulkier aluminum part (yay me).

You do have to address fatigue issues with aluminum, though, as it has no fatigue limit stress (stress below which you can run infinite cycles without developing cracks), this has to be taken into account in the design as well. But generally, for many types of structures, you can make them lighter-weight with aluminum. But there are ways to make steel structures pretty lightweight as well (Ducati sportbikes, S2000, FR-S/BRZ, etc.). There is no one *way*.

 

Sometimes I wish I could go back to structural design. Managing pharma projects certainly does not jog the brain the same way.

 

There is certainly more than one way to skin a cat, but much more limited when you're on a budget and need to turn a profit.