Re: Bridge vs. Neck pick-up ...
If you're talking just Seymour Duncans, yes, it's based on resistance rating. The neck models are underwound to balance better with the bridge model and give a little more clarity. If you wanted a more correct PAF setup you'd use two bridge model pups, as there were no separate winds back in the day, nor was there any measuring to see which was hotter; you could easily find the neck pup hotter than the bridge pup.
Modern Gibson pups which have the "R" and "T" suffixes have different pole spacings as well as winds (the bridge has wider spacing). The 57s and Burstbuckers are similar to the Duncan PAFs-same spacing for either position and separated only by their amount of winds/resistance.
Dimarzio has their standard spacing, and F-spacing, which is similar (but dimensionally different) to what I described about the modern Gibsons.
