What To Look For
The standard rule is that steel is twice as strong as aluminum, but at the cost of being three times as heavy. Ergo, if you are looking for a light bicycle for cross-country touring, a cruiser bike to ride along the beach promenade or around the neighborhood, or a road racer, you should choose aluminum over steel most of the time. A bicycle that takes a pounding, such as a mountain bike, should be made of steel.
The steel vs. aluminum picture is complicated, however, because not all steel bikes are made the same. A steel-tube Schwinn from the 1960s might be a heavy beast, but not a modern bike made from high-quality steel and employing advanced engineering to shave a few ounces here and there. Such modern, cutting-edge steel bicycles are very strong, yet are almost as light as their aluminum counterparts.
Where To Buy
If you can determine your bicycle size and are not intimidated by assembling a bicycle at home, shopping online offers a wide product selection and improved bargain-hunting power. If either picking the right size or assembly is a problem, go directly to a brick-and-mortar bike shop.
Excepting the cheapest steel bicycles made from inferior metal, aluminum bicycles will always be cheaper. However, it is possible to find old steel-tube bikes from the 1960s and 1970s, owned by people who do not realize the value of what they have. These bikes sometimes go for as little as $10 or $20.
Because aluminum is weaker than steel, it suffers from metal fatigue more profoundly. Depending on how often you ride the bicycle and how harsh the conditions are, an aluminum bike frame will last only five to 10 years before it starts to show signs of wear. Steel can last for decades but requires periodic refinishing to avoid rust.