I've got a new Champion Flyer, which is pretty much a B-17 with springs. It's breaking in pretty well after about 150 miles. My first ride went pretty well, by the way. The stock was getting uncomfortable; the Brooks was also uncomfortable, but in a different way. In any case, I prefered the Brooks, right out of the box.

Can't tell you how to get the right fit, but wallbike.com says they have fewer returns on the B-17 than any other saddle. Also, Sheldon Brown says that if you ride more upright than otherwise, a model with springs may be the better choice.