well I tried underdrive and it caused problems. faster revs, more responsive but it caused problems that nobody else had.. whatever the reason, both of the pulleys will help you gain more torque and power all over the range as it's not related to air or fuel.. underdriven will give you faster revs, hp & tq, stock size will give you faster revs.. if the whole underdriven set, it will put more strain on the alternator and if you're favoring big subs etc, forget about good electrical power..
In my case, I believe it made the water pump run faster, making an annoying sound as well as affecting something else to make another annoying sound.. Hope you don't have these..