I do both because I don't think you can dissolve sufficient O2 in a one or two qt starter for a five g batch. I have no problems with this, but if some knowledgeable person convinces me otherwise, I may change my mind.
I do both as well and I'm happy with the results. One tip though, always have a backup O2 cylinder and take off the regulator when not using it. I left mine on and when to aerate wort one day and pssst, then nothing. I now always have a backup cylinder, just like I do with propane.
It makes for a bummer brew day when both run out and you are not prepared.
Is the real O2 really that much better than pumping air into the mix? I would think it has the same effect it just takes longer. I'm on a budget and I'm leaning toward the cheaper air pump with filter and stone setup. What do you guys and gals think?
I believe the answer is that air is 20% O2, so this limits the maximum oxygen you can get into the wort no matter how much air you pump in. Whereas O2 from a bottle is 100% O2, so you get five times the oxygen saturation.
I'm not convinced that O2 injection is really an improvement-- but hey, I'll try it and see