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That Sky and Telescope - Four Myths article is a good read and debunks some of the fanatical rants we see on here from time to time. Pyrex vs BK7 is a good example, and of only real value in marginal improvements in cool-down times for the people making the mirrors - not significant to end users. Much of the noise about mirror substrate is nothing more than marketing efforts to differentiate a product to put a premium price on it. Also Pyrex is a brand itself and thus attracts a premium because of that. I have heard, but can't say it is proven, that Taiwan and Chinese made mirrors can not access Pyrex stock because of trade restrictions and /or price premiums put on by owners of thd Pyrex brand and thus favour BK7. Taiwan maker GSO turns out a very nice mirror for the price and sell these for scope making. Royce, Zambuto, etc are premium brands that differentiate themselves on design (Royce conicals for example) and/or substrate brand, figure and finish (Zambuto). You choice needs to take into account your end use, for example an imaging scope can perform great with a mirror the visual guys would shun - horses for courses.
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