If it were me the decision is close enough that I'd probably leave it to the surrounding details of each transaction. I'm a particularly snobby and self-absorbed curator of "builds" so that would be a huge vote in favor of the 700 because it's a frame, unless the curator of the 650b build did a lot of the things I would have done. The effective "labor cost" of doing a build is essentially zero for me. Is one a shop and the other a person? That would have some weight for me. If the shop was one that I could frequent, putting money in the register of a shop reaps benefits down the road. If the person was one that I had particular respect for, that could tilt me a different way.
Then there's color!
but...if we took it to a purely hypothetical situation, and asserted they were both frames, both in identical condition, at the same price, and from the same seller, so the real and only difference was 650B vs 700, then I'd probably lean 700 because the 700 wheel on the 54.5 is "the latest", and probably has a tiny bit better future proof resale value. If that wasn't the case and it really was completely wheel size then I'd personally probably lean 650B for a Hilsen, but it would be like a vote of 52 to 48, and my leaning may change on another day.
Both are great bikes.
Bill Lindsay
El Cerrito, CA