Due to the derecho, we were getting 11 windows replaced with Pella fiberglass windows. Our contractor uses them (Pella) exclusively, which is fortunate because 4 years ago we replaced all 9 bedroom windows with those. We went directly through Pella and paid about $850 per window, installed. Fast forward to now, and we're getting the exact same windows at cost...$400 each, installed. Naturally, we're taking advantage and getting the additional 9 replaced, so the entire house will have new windows.
As far as fiberglass goes, they seem to work best in our climate because they're less likely to expand and contract with the heat and cold. Overall, we like the Pella windows. I wouldn't pay $850 per window again, though.