Most bikes/trikes are built on either steel frames or aluminium frames, and which one you pick will impact the quality of your ride and the life of your trike.
Aluminium frames are generally stiffer than steel, resulting in a harsher ride. When fractions of a second count, track racers prefer that rigidity. But for getting to the store over city streets or traveling around the world, steel offers a more forgiving ride.
No frame material is more durable than steel. It can ding, scratch, dent, even bend, and retain structural integrity. When well made and cared for, steel will last a lifetime.
The same isn't true for aluminum. Aluminium is more brittle, so it's more susceptible to failure through fatigue or damage. A single dent in the wrong place could render an aluminium frame unsafe to ride. And when aluminium fails, it fails catastrophically.
Think about it like this: glass will break before it bends. The same is true in many respects of aluminium. Steel on the other hand, will bend before it breaks, resulting in a far more resilient frame.
STRENGTH to WEIGHT:
There's a misconception that steel bike frames are heavy, and aluminium frames are light. Not surprising, we make bridges out of one, and soda cans out of the other. And while it's true aluminium is lighter by volume as a metal, that property doesn't tell us the whole story.
To make aluminium frames strong enough, builders often make the tubes larger and thicker than steel tubes. That extra material adds extra weight, and quickly an aluminium frame can weigh just as much or more than a steel one.
There's a saying in the bike industry, "steel is real". Steel isn't the best choice because our frames are made of steel. Our trikes are made of steel because it's the best choice.