I have both shoes in a mountain bike type. I have been switching between the two lately, and have noticed some differences.

The Sidis conform to the foot more, and I do find I pedal more efficiently with them. These are not carbon soled like my Specialized. Thus, the benefit of carbon soles may not be as great as I thought.

The Specialized shoe is more roomy, and does not squish my foot as much as the Sidi.

Bottom line. If I could find a shoe with a fit just in between the two, that would be perfect!!