| The difference between neck and bridge as far as the construction affecting tone is negligible. At least in my experience, the difference between a neck and bridge pickup from a construction standpoint is all in the width of the pickup. Bridge pickups are often slightly wider than neck pickups because the strings are spaced wider. However, many Jazz bass makers have stopped doing this because they either use blade pickups that don't use polepieces, or because it quite simply doubles production cost for what turns out to be a negligible difference in sound (as long as the string vibrates within the two polepieces, the difference between having them exactly centered and slightly off-center is so minute as to be undetectable by the average listener).
If your desired pickup brand has different models for neck and bridge, make your decision based on the width of the pickup you currently have. Putting a neck or a bridge in there will sound the same, because the difference in tone comes from pickup placement; the neck gets more of the fundamental while the bridge has more midrange and treble. | 
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