Ah, if we are talking about mountain bike frames, then materials, weight and durability may be more critical than for road bike frames. I have a Specialized hardtail and KHS FS mountain bikes, and both of them are 7005 aluminum alloy frames. Just based on my riding, my MTB take much more of a pounding than my road bikes, so materials choice and durability are more important for the MTB. I wonder if an aluminum/scandium alloy MTB frame is stronger than a 7005 MTB frame of the same geometry. I have done some brief looking, and have not yet found a scandium vs. 7005 MTB frame of the same geometry, to compare the frame weights.
Honesty compels me to add that with my style of essentially recreational riding these days, a few ounces plus or minus on the frame makes no difference to me. If I was racing for money, it no doubt would.