I guess it really depends on your riding style, how weight consious you are, your budjet, and how you want the bike to behave.

Let's say your a hardcore XC racer, you want something really light and stiff, so all your power goes to the ground, like carbon or aluminum. Or if all you do is urban stair jumping, then cromoly is nice and compliant, but cheap to replace or fix if you crash.
Let's say your more of a not-so-hardcore xc racer / trail rider, a ti bike is light, but has some compliance so you can ride longer without vibrating off the bike.

Hope this helps

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