I went through this same (yet somewhat different) thing, I originally bought Goodmark 1/2 quarters and then later decided given the amount of work and money I had into the car, the car deserved full quarters. I looked at Goodmark and AMD at a show and talked to a number of people; to me the AMD were far better in overall quality. Every professional builder I have talked to recommends AMD. I have good mark fenders (because AMD were not available) and I can tell there is a difference in thickness and overall quality. All that said, Goodmark are not super terrible, just not as good. You will spend more time fitting and straightening the Goodmark I believe. I have not put a lot of quarters on, however I will say up front that it isn't a quick job, you need to spend a lot of time fitting, trimming, etc. I think I test fit each side at least a dozen times to make sure it was as good as it could be and I am sure I was still premature a little.
The guys who do this for a living on here are better judges as they have most likely seen both up close, but my vote is obviously AMD.