General question on Carbon fiber tonearms versus aluminum

It is not necessarily the material but the execution of the material that defines the performance. Straight aluminum, tapered aluminum, stepped aluminum, foam injected aluminium etc etc.   

Another thought is aluminum may sound better with MM cartridges and CF with MC cartridges.

@slaw Why would one material sound better with MC cartridges than MM?  

@testpilot Agreed, although  I was thinking more in general terms.  That is, it could be easier to make a better sounding tonearm with CF than AL.  Clearly, it is that way with bicycle frames, it is far, far easier to make a frame stiff in one direction and flexible in another with CF, or make it a more comfortable ride with less vibration than say AL or even steel.   

I'm not sure that either aluminum or carbon fiber is inherently better than the other, but magnesium is certainly one of the best materials for a pickup arm - such as is used by SME and Technics on some of their arms.