Commute distances are generally short enough that I don't think it matters very much. I do like carbon because it can be made compliant and does a good job on rough roads. The problem with carbon is the price. I also agree with the person that said low end carbon is not the same as the high end, even though they look almost identical. Vélo News had an article where they compared a Chinese carbon frame sold for $700 to a Cannondale Synapse frame, and there was a big difference.