Lead core is very speed sensitive... a change in boat speed, wave action or direction will cause the lead core to rise and fall in the water column. The shorter the mono leader, the more rise and fall the lure has.... longer leaders will mean less vertical lure movement. I like the shorter ( 20-30ft ) leaders so my baits move up and down a bit. If I want to hold at an exact depth, I will run a long lead off a downrigger.
Just my $.02
Jim S.