With a good shaking (actually it is more of a "rocking" with a full 6 gal carboy), you should be able to get 4-8 PPM of O2 in solution. With a stone, you are looking at 14-20 ish PPM depending on the flow rate. Both will give the yeast what they need to get going. After you pump the O2 into the wort, the O2 in solution will come to a static equilibrium with atmospheric pressure. If you do not keep adding O2 under pressure, you will get about a 3-5 PPM equilibrium in solution due to pressure equalization very rapidly (just like soda will go flat very rapidly when left exposed to normal atmospheric pressure...the excess CO2 "jumps" out of solution...O2 will do the same thing). Most (if not all) beer yeast can do both anaerobic (without O2) and aerobic (with O2) metabolism, they just produce slightly different compounds under each condition.
I have done an experiment when growing yeast...constant stirring vs just letting it sit in the flask. I can tell you from empirical observations (I did not have my microscope at the time, so no cell counts were done) the constantly stirred (aerated) starters of the same yeast and yeast food, did much better in overall formation of yeast cake. Now was this due to the aeration or the fact the stirred yeast cells were kept in suspension and exposed constantly to food and nutrients instead of being allowed to settle to the bottom (being "lazy")...I don't know. I am leaning to the "they are in solution" theory and not being allowed to be lazy critters at the bottom of the flask. More research needed.
Last bit of empirical evidence. On several brewing sessions, while doing a 12 gallon batch of beer (6 gallon transferred to 2 carboys) I have forgotten to turn the O2 valve ON. I usually figure this out during the transfer to the second carboy ("hey, where are the bubbles at...Oh Crap!"). So the 2nd carboy gets O2 and the first one gets the @#$% shook out of it. The beer hits the same FG at the same time and there was no taste difference the couple of times this has happened. I have never had a stalled or slow fermentation since I started brewing.
If you are building a rig and have the $$ for an aeration system, they are cool to have and eliminate one more variable in the brewing process. It is not like most home brewers are doing 400 gallons batches at a time...Andy excluded...and are concerned about surface area diffusion of O2 in large static volumes...so the levels of O2 in a 5-10 gallon batch is not that critical (in my opinion).
The size and health of the yeast pitch, on the other hand, will have a huge affect on your final product.
Just my lengthy $0.02...ok, that was more like a $1.50 worth
Jason
