Fiberglass will cost more, but give you a superior result for keeping the house cool in summer / warm in winter. If you can afford them up front, you will save more in the long run if you plan to keep the house for 5 years or more. Do consider your window profiles (style and muntins) so you do increase / maintain the architectural value. Many homes of this vintage have had the original muntins / style and profile of the windows replaced once in the 40's 50's and lost all the original charm. If you want assistance, please post an elevation photo of the front or rear or both and we can help you with that too.

Vinyl windows have come a long way - depending on your neighborhood, budget and value of the home, they may be the right answer. 40 windows is a lot to replace and yet it will make an enormous difference in the comfort level of the home and its annual energy usage. Don't switch to picture windows with no ventilation - cross ventilation is critical to a foursquare . .