Well for me, I would certainly go for the 'B' along with the race frame. Probably the pro's with their experience in low wind would disagree but for the rest of us the 'B' is the business! IMHO.
I'm a bit late to this party, but looking at the start of the thread it seems to indicate that a B series will be just as good as a 1.5 SUL. Would that mean that a B series would perform better in low wind than a 1.5 SLE with the same frame?