I noticed Oatey has a inline vent that the big box stores carry.

[www.oatey.com]

What is the difference between this and the much much more expensive AAV vents by say Sioux Chief?

The inline black vents are cheap and wear out quicker and make lots of noise and have a low dfu rating. The expensive ones last longer and quieter and some have a higher dfu rating.

Price and quality.

The in-line vents are cheaper, but don't expect them to work.

i prefer the Oatey to the Studor's.....they are less expensive and heavier...ive never had one fail on me yet(that i know of)

quote; ive never had one fail on me yet(that i know of)

Even when they fail you may not know about it unless they complain about sewer gas odors. IT's not like a red flag pops out and says "change me".

King AV200 Drain vent --- This King model looks VERY similar to the Oatey vent on OP link.

Two of above King AV's installed aprox 1980. In 34 years sinks drain and no sewer oder ever detected. At that time this was the only model available in the local hardware store. There was no Internet for research comparison. I bought one extra for possible replacement. It is still packaged and in my garage.

I suppose if I were to install AV's in a location that I could not personally monitor, I would compare all AV's now available for any possible quality advantage.