I've been reading on that too, just for a short explanation:

UnderDrive Pulley
-Same size as stock usually
-Slows down alternator so frees up some drivetrain loss, which basically means it frees up from 10-15bhp, and possibly 5whp.

The lightweight pulley
-lighter then stock pulley
-Since it's lighter frees up some drivtrain loss, and frees up your hp same as underdrive, but w/o slowing down your alternator.

Just FYI: Since the underdrive slows down your alternator, it wont charge your battery as fast, so if you have a good soundsystem and not using a capacitor, you may need one, or look for an optima yellow battery.

Basically I'm forgetting about the the battery/pulley because of an article I read, the consequences/results far outweigh the hp gain to the wheels. I'll post the link if I find it if you want...