Ah the old Aluminum vs. Steel debate. I have a somewhat unique perspective on this as I have rode both, (thousands of miles) and I am huge.

I don't think you will break a well made Aluminum frame I have road my Cannondale Killer V 1997 (I think) for about 30,000 kilometres, and it has not broke. What it has done has at times is loosen my fillings. The tubes on the beast are huge, ( I am willing to bet that the Killer V frame is the stiffest bike frame every made) it does not flex.

So then is Aluminum stronger then steel, no way. Al. bike can be stiffer because they can use much larger tubes without being heavier. I once ordered a Rocky Mountain Hammer Race in a XL. When it came in I sat on it and torqued the chain stays almost an inch sideways. (I didn't buy the bike).

Can steel bikes be stiff, strong, and long lasting? Sure you just can't worry about weight, check out the size of the steel tubes on my new bike, steel frame (the frame weighs about 5 pounds). I think what happened to steel bikes is that the manufacturers got caught up in the "weight weinnie war"and gave up stiffness to reduce weight.