I have been trying to answer this same question. It is really a pretty simple concept, a plastic cube with one end missing, attached to you hip (belt or paddle). The only way I could see the more expensive models being better is IF they have a tension adjustment, AND you used it enough, to wear it to the point that it no longer held the mag securely. I really don't see this as a problem though.
Myself I went with Fobus because I found them for ~$15 and they didn't have an Uncle Mikes in stock to fit my mags. I guess in Cali $20 probably isn't too bad. I do use an Uncle Mikes holster though. From what I have seen the "tacky" compound of the Fobus leads to more wear on the guns finish.
Edited to say - - I didn't know the tension was adjustable on the Uncle Mike's. Shows how much I know.