"Quality", as in the general roundness, straightness and detail of the tube manufacturing is generally going to be similar. If you mean "ride quality" then don't worry about the alloy. In your price range they are all going to be made out of similar steels- it's what they do with the steel that matters. Frame design, fit and features are going to have a much bigger impact on the bikes appropriateness for you then what pipes are in it. Go test ride some bikes and take home the one that meets your needs and its your body. Don't get to hung up on (or even rally worry about) what the little frame material sticker on the seat tube claims.