I think the reasons AGS are more expensive have to do with the fact that AGS puts "Ideal" cut specifications on their reports, and GIA does not do this FOR STONES WITHIN CERTAIN CUT PARAMETERS. Thus some consumers are LED TO BELIEVE they are getting a better diamond if it is AGS. Consequently they might pay a little more.
However, months of research and market study leads me to believe that GIA EX/EX stones with Sarin reports that indicate AGS CUT GRADE 0 proportions might as well be identical. The theory is that you could send such a stone to AGS and get a CUT GRADE 0 rating. Cut is cut is cut, and the actual numbers aren't made up by GIA or AGS, they just report the information (or some of it).
You may notice you won't see a lot of AGS stones for sale THAT ARE ANYTHING OTHER THAN AGS CUT GRADE 0. It seems evident from this fact that vendors CAN GET MORE MONEY FOR AGS CUT GRADE 0 stones, BUT NOT for AGS CUT GRADES 1 or 2, grades that could still be covered under a GIA EX/EX grading report. If you were a dealer, you'd want a GIA cert in this case, I suppose.
When I buy my diamond in a couple of months, you can be sure I'm not paying as much attention to whether it is AGS or GIA, BUT WHETHER THE SARIN DATA AND THE LIGHT RETURN DATA ARE IN THE RANGE I WANT. At the end of the day, all I care about is how the stone looks. To be sure, AGS CUT GRADE 0 IS NO GUARANTEE OF THE BEST CUT STONE. There have been many examples here, even recently, of not so nice AGS 0 stones. One had a 64.1 depth and IS NOT what most people here would consider to be an "ideal" cut, even though AGS said it was.
I'm of the firm opinion that GIA EX/EX or even VG/VG stones with certain PREFERRED PROPORTIONS AND ANGLES are equally "ideal". It ain't the grading report that makes the stone. It's the cut. Why pay a premium for AGS 0 if you don't have to?