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The expansion/contraction ratio between the fiberglass and the glass pane is not important. The seal between the stiles and the windowpanes are rubber and the glass can expand and contract at a slightly different ratio with no adverse effect.
Fiberglass windows do have a few advantages over vinyl but they are not of major importance in my opinion. My philosophy is if you can afford them use them, if not don’t. I don’t think you will regret your decision.