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Pyrex is better but either will do the job. I'm very impressed with how almost immediately usable my 10" Dob with the Pyrex mirror is. Also as temperature fluctuates throughout the night a Pyrex mirror will be less affected.
My 10" pyrex is less demanding wrt cooling than my 8" BK7. I have fans behind both though. Pyrex will help the mirror maintain its ideal shape. It will not help with inversion layer and tube current. So you really need a fan for that in any case if you want your scope to perform at its best.
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