Shell is slightly looser than calf, but not much. I have 40 shell models: mostly shoes; some boots. But that's not what was asked. What was asked was whether he should go to a B width or a D width - and the question was asked in light of the fact that he is a C width in the PA.
His question/comment was: Hmm so I am a 10C in Park Aves... what do go in 5th street cordovan (only B and D widths available...). IIRC people say cordovan fits slightly tighter than calf? So maybe the D...?
And my answer was: You risk the B being unwearable and the D being slightly loose, yet wearable. I'd go D.
I'll stick with my answer, but it goes without saying that both widths should be tried on (or ordered) to determine the best fit.