I think the Airmax is almost identical to the Vantage Max, except it has a different reticle.
The 3-9x40 for 」75 seems very good value. You would be limited to 9x mag, but most of us only use 10x mag anyway, so it wouldn't actually be much different.
The Vantage Mildot (NOT the Vantage Max) has a much thicker reticle, and doesn't have the half mil dots. I personally prefer the thinner reticle on the 'Max' version. It would be worth getting to your local club and having a look through a couple to see what type of reticle you prefer
A more expensive version (such as the Sidewinder) benefits from better glass and more coatings on the lenses. This should give a clearer image in a wider range of conditions. Unfortunately I haven't had a chance to use my Sidewinder much and I don't want to change scopes at the moment. Maybe in a few months I will put the Sidewinder on and see how it compares to my current scope