we have two large windows also, 4'x6'
Vinyl is a popular choice for one 2 reasons. First is cost, although vinyl windows come in a range of costs from inexpensive to expensive. Second reason is for the energy saving properties of sealed double pane windows. With vinyl windows as anything, you will generally pay for what you get. So if you pay more, you will end up with a higher quality window. Dont just buy vinyl without shopping. There are many choices and you want to get the best for your money if this is for your own home. Fiberglass windows will follow along with vinyl windows in my recommendations. So consider what you want from your windows, decide how much of a factor cost will be and then shop around. Good luck with your project. One last word, make sure you get professional installation.