The question of steel vs aluminum is one that has been asked for a very long time. The answer has to be only that it really depends on the project.
The question of which is stronger has come about due to a total misconception. This misconception has derived from the partial truths perpetuated that there are aluminum alloys that are as strong as steel and this simply is not the case. It is better to say rather that there are a few aluminum alloys that are stronger than some steel alloys.
6061 with T6 temper is one of the most used aluminum alloys and it is definitely stronger than some steel alloys pound for pound. The truth is that no matter what the strength of the aluminum alloy you can always find a steel alloy that will be stronger.
The fact of the matter is that other factors will determine whether you should rely on a steel or aluminum alloy. It depends mostly on the application. What you are going to use it for, is the deciding factor.
Let's look at some other comparisons between aluminum and steel. Look at the flexibility issue. There really is not a stiffness advantage here. Strength of a metal refers to the maximum load that it can handle without buckling. Stiffness is in reference to how much it will give before it breaks.
For instance a camera on a boom needs to be stable and steady. You need the metal to be stiff not flexible. And a tower needs to have a certain give to withstand high winds. Stiffness is quantified by the Modulus of elasticity and aluminum it is 10 million-pounds per square inch, whereas in steel it is easily 3x that amount.
As far as weight is concerned because of the thickness that the aluminum will need to be to be comparable in strength to the steel, it will weigh relatively the same.
Metal fatigue is usually not a consideration for a general structure made of steel. However if you are making engines and plates from this material it then becomes of greater importance. Aluminum is subject to fatigue failure. It will reach its endurance limit long before a steel hull, plate or engine. Today these too can be compensated for in the design of the engine, vessel or structure. Unlike steel aluminum has a very low rate of corrosion and this must be considered in the final analysis.
There you have it. The steel vs. aluminum saga will rage on but at least now you are aware of the factsthat it is a matter of suitability over strength.