Im not sure where that saying comes from but I imagine it's roots are questionable.
Comparing 2 bikes based on material alone, especially with one of them being a complex composite and one being an alloy, is apples and oranges. A lot of people think metal is stiffer than plastic, so it must be better, but its really not that simple. Aluminum also has a pretty low fatigue limit (the more stress it cycles it sees, the less it can tolerate), where carbon fiber, generally speaking, maintains its tensile strength very well. Of course, its a composite material and there are a million influences on its structure, so you really cannot compare the 2. However, pound for pound, carbon fiber 'should' always be a better material that alloy (assuming better = absolute stiffness).
My point is what would make one option better than the other? A stiffer BB? Crash resistance? Weight? Cost? Of course arguably the MOST important question would be which one fits you better, as that is the better bike for you
