I believe it's all about what happens at the surface. Airstones, as mention, do help with current and movement in dead spots. Which is great...The small bubbles, while under water, however do little to provide oxygen. They, at that point, just look neat and of course move water.... But all gasses, co2, oxygen, etc rise to the surface of water in tanks...the rupture of the bubbles at the surface produces surface agitation that helps with the expulsion of co2 which can be harmful to fish in great amounts ( but also promotes plant growth )... Additionally, those of us with hooded tanks, benefit from the additional movement and release of stagnant air between the glass top and water's surface. For this reason, the more surface agitation, the more oxygen becomes present as it is circulated under water by its pull and eventually drawn up top prompting the increase of release of other gasses such as co2... So for aeration, a powerhead and/or spray bar positioned correctly is superior - but if in a planted tank the on going rapid release of co2 (which was recently pointed out to me here at TFK) may negate it being supplemented in traditional co2 setups.... So as far as surface agitation, you just got to play with it, till it meets the needs of all the living organisms in ur tank....some fish do like more stagnant environments like swamps thus benefitting from airstones solely...other enjoy faster currents commonly assoc with streams and rivers...thus power heads.....