Generally you would get one or the other. Either get the underdrive accessory pulley kit (water pump, power steering, and/or alternator), or you can go with the crank pulley. All this does is slow down the rate your accessories run at, and spare you the little hp that the accessories were using up.
Both ways are not dangerous at all, but if you have a crazy sound system, then you dont want to underdrive your alternator pulley.
Also don't expect to feel any difference from them.