Personally, I trust Costco's quality and customer service (as far as backing their product and refunding money if there are any problems) compared to America's Best. A private optometrist may give better service and possibly better quality, but you will pay more money for that.
You can read more reviews on America's Best in this thread. As to the differences in thickness, at your prescription strength there isn't much difference between 1.67 and 1.74, maybe a difference between a 5mm edge width and a 4.7mm edge width. If the price is almost the same, it may make sense. The optician may be able to give more accurate recommendations.