IIRC from my undergrad materials classes, aluminum is problematic in that it does not have a fatigue stress limit, whereas most other metals will plateau after a certain number of cycles. The obvious solution is over-engineering the frame, but at a weight penalty. Interesting case study the prof had told us about, a lot of early passenger jets were high-wing, but passengers would look out the windows and have a panic attack when the wing skin would be cracking under lift load...

I agree with some other points made. Carbon is probably not a good choice for toughness on MTB's. Steel is indeed prone to corossion, but a good powdercoat and the most basic of care is usually more than adequate.

Regarding how well the bike does in a crash...Cars are designed to deform in such a way to absorb as much impact energy as possible, so your body does not have to. Wouldn't that apply somewhat in a bike collision?

Ti does indeed seem to be a good choice (esp for MTB's), based on corossion, strength, durability, fatigue life, etc. But what about stiffness and comfort? Price premium over carbon?

I did go with CF for my first new road bike I should be picking up tomorrow, because I've heard time over time it makes for a relatively comfortable ride, which is important when trying to finish a century or put in big miles. MTB riders aren't in the saddle nearly as much, so comfort probably is not as important?