In the past, top weight gave you length and backend while moving to bottom weight gave you an earlier, more controlled reaction.  How true that is now is a different story.  Throw modern dynamic cores into the mix, throw a dizzying array of coverstocks into the mix, throw a core with a spin time in the 5s range into the mix and whatever effect the static weights once had is almost negligible.  

Assuming you can throw the ball consistently enough to notice a difference between two otherwise-identical balls.

SH