I know I'm comparing apples to oysters here, and I don't have a clue exactly what I'm looking at....with that preface out of the way....
I just watched videos of the Huntfield Derby and the 2012 International Hunter Derby...I'm not so sure the more traditional set have anything to worry about as of yet until the more appendix-type AQHA's get out of the strung out Western Pleasure look, drive off the back end and get lighter on the front end.
There are people who do AQHA shows who could easily cross over with some of the horses and do well at USEF shows. The AQHA Derbies look like lots of fun. They seem to give out good prizes in addition to the money, and I believe they recognize the highest placing Non-pro, etc. The courses look great. I think they even used the squirrel jump last year when they had a Derby at the Kentucky Horse Park!
They have a few different fence heights at the derbies depending on the class. I was so excited to see that they are doing it, and yes the prizes offered were wonderful! Some of these QH's do hold their own at USEF shows.
I show my AQHA gelding at both USEF and AQHA shows. He holds his own on the 'A' circuit, but we do the AA, not the AO's. Generally, people at the USEF shows are surprised when I say he's not a warmblood. I am proud of the direction AQHA hunters is going and I'm proud to have a QH. I hope they continue to move forward to dispel myths about the breed!
The hunter derby is a new idea to the AQHA crew, but they are embracing it and trying to make it better. Lainie DeBoer and David Warner founded the series and have been instrumental in it's success.
As to the heights - generally AQHA Amateur and Youth Working Hunter is 3' while Senior Working Hunter is 3'6" (that is the open class for horses 6 years and older). Most of the Derby classes are 3'.
I rode an AQHA hunter who would certainly have held his own in an open show. Although the way they trained him for the hack was ridiculous, he could clean up over a course. You had to push a bit to get the striding, but you don't notice that when watching him. Their H/J classes are looking better! I was very impressed by that video.