You are going to get a number of opinions on this. I believe a clad wood window with low-e glass can't be beat. Not only is the glazing system efficient, but the frame does not transmit cold. The wood core is extremely stable preventing window seals from breaking. We have Andersen Windows throughout the house and have never lost a seal. They are guarenteed 20 years against fogging. Look for warranties, and check the manufacturers U ratings for the same size glass, but remember, Glass is only part of the window system. You can't beat the appearance of wood windows trimed in casings and sills indoors. They are warm, quiet and beautiful. Downside is they cost a great deal more than contractor grade windows and do not significantly increase the resale value of a home.