Originally Posted by
cyccommute
The last half of your post is just about right but the first part is dead wrong. Aluminum is soft and can be dented but it's no more prone to dents than thin steel or any other much thinner metal. If anything, aluminum has a thicker wall than just about any other material to make it more dent resistant. There are literally millions of mountain bike frames out there that are made of aluminum that get dropped, crashed and abused daily without issue.
I think you're both right and wrong at the same time. The top tube of my (scandium aluminum) road bike has such a thin wall in the middle, it looks like a soda can. It almost seemed like the wind could dent it, and a wood box falling on it ended its life. My mountain bike, however, has been dropped multiple times without a dent. So, it depends on which frame you're looking at, and how thin they make the walls.
Steel is the same way. If you're looking at "oversized" steel tubing, you're probably looking at something with thinner walls. If Reynolds 531, not so thin. Even then, some tube sets are butted and some aren't.