I think that the real truth is that most name brand filters probably work reasonably well in most applications. However, I have no problem stating my opinion that certain brands like Wix, Mann, Mahle, Purolator etc. seem to be of *better* quality than others, such as Fram (which has had known problems in certain applications.) When the prices of all of them are so similar, there's really no reason not to buy the better quality unit.
Now air filters are a lot simpler than oil filters (which is where my opinions of brand quality really come from) so realistically, if an air filter appears to be good, it probably is. I'll use Fram air filters for convenience reasons where I wouldn't touch their oil filters with a ten foot pole. However, given the choice, I'll grab the Wix or Purolator box every time.
nate