Bridge pups are generally have more output and maybe more bass & mids than neck ones IN A MATCHED SET. *This is because the magnetic field generated by strings at the bridge is weaker (and brighter sounding). *But if you know the characteristics of the pups you're interested in, you can make some educated guesses as to how they'll match up. *Or, you COULD just go for it. *You can always change it back. *
Some pups work well both places, but they're usually vintage or pseudo-vintage, "all around" designs. *More specialized ones tend to not be as flexible.
Check out the charts at
http://www.jemsite.com/axes/htm_features/det_pu.htm