I think the larger issue is that manufacturers don't generally equip their full carbon offerings with mount points for these things vs a alu/carbon debate. If you had say my uptown in carbon & alu, then you could compare apples to apples. Till then, I think most carbon frames you'll find are not commute minded, rather race/fitness minded.
That's my thought on it.
if you do find a carbon bike with mount points, how would it compare spec to spec to an alu equivalent might be the best question.
- Andy