Replace windows or leave the the 1980 versions alone?

[Jay Helms]

My investment partner and I are under contract on a 42 unit apartment building. Built in the early 80s and very little to no capital improvements since then, the property currently sits at about 50% occupancy. As we're working through our renovation budget, we're torn on the windows. Currently single pane, metal frame, very non-energy efficient (tenant responsible for electric bill). Question is...will replacing the windows attract a better tenant at higher rent or should we leave as-is?