Okay [thinks back to 3rd form science class] the way I understand it is this:
Gasses are extended between air and water wherever they come into contact with each other. Therefore, the more water moves the more gas that can be exchanged. (This is because more water is spending time on the surface.) While an air stone causes water to come into contact with air the exchanged gasses still have to go somewhere. [This is why bubbles go up.] The bubbles going up causes water movement. – This means that more water spends its turn on the surface in a shorter period of time, which allows more gas exchange. While a minute amount of gas exchange happens with the small bubbles from the air stone the majority of the benefit comes from water movement.
That said, my UG outlets are above the water surface causing a waterfall effect. This is simply because I like the sound of waterfalls better than the bubbling of hot pools and when you have a *few* tanks running you need to consider sound as well.
Hope this helps.
KA.