Aluminum or steel? It depends on the application, really. Aluminum has a fixed number of cycles before metal fatigue sets in that is far lower than steel. Steel has as far as the stress of a bike frame, nearly an unlimited number of times for the stress cycle to load/unload it. Aluminum is lighter, and will break easier.
Steel is easier to repair and work with than Aluminum. It's more a matter of personal taste, though.
__________________
. “He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you.”- Fredrick Nietzsche
"We can judge the heart of a man by his treatment of animals." - Immanuel Kant