Low e glass or double pane glass for replacement windows?

I have over 30 years experience in the glass and window industry. The best window glass is a triple-pane argon filled Low-e unit with the coating on the 2nd surface. This is quite costly but is highly efficient. If you want economy but decent performance, the thicker the spacer bar between the panes of glass the better the performance. 7/8″ is generally the thickest unit for residential widows. Add Low-e and you have a good window. If it is real sunny in your area a tinted glass unit works well too.