I'm having the same headache at the moment.. from what I can gather from Sheldon Browns website, the larger the diameter of your seattube, the longer the axel is required to allow for the throw of the front mech. So I'm currently looking for an XTR BB, for my Kona Kula with a 27.0 seat tube i'd get a 112.5mm axel, but for my Clockwork with a 29.6 seat tube, i'd look at a 116mm axel. I don't know if this is completely right, but it seems to make sense.
it all depnds on the cranks you are using . from early to mid nineties , cranks were straight , so whatever your frame you needed a fairly long axle ( 122 plus ) .
from the mid nineties , cranks started to be curved , so bb axle were shorter .