In addition to the lower mass, another major benefit of the aluminum-based cone is its higher rigidity, i.e., higher internal velocity of sound propagation. This means that the cone moves uniformly (without "breaking up") over a wider frequency range. A potential drawback of the aluminum cone its low intrinsic acoustic loss, or high Q around the upper-end resonance frequencies. They usually need some kind of laminated aluminum materials to "dump" the resonance (I don't know what exact treatments the Axiom cones receive).
I believe that heatsinking ability of the aluminum cone becomes fairly significant when the speaker is driven at high average power levels.
