Re: Which U-Factor
Insulation value also doesn't tell you much about air leaks. Better quality windows may have the same R value rating, but will seal out air better and continue to do so for a longer time.
I know I'd put my 90 year old single pane wood frame windows up against many newer vinyl replacement for tightness. I can maintain a dewpoint indoors of 50F, when the dewpoint is 72F outside. Keep in mind there's no vapor barrier in the house and I have a LOT of windows. 42 mostly large windows in a 3200 sqft home.
IF cost is an issue. IF you have some common size window frames, you could install and re-use several storm windows for a short term solution, then replace a couple at a time and play musical storm window.
1925 Two-Story Stucco Beaux Arts Neoclassical