Option 1: Restore the original windows and replace the storm window with a low-E coated storm window.  The existing windows are assumed to be around ±R-2 (single glazed with storm windows). The fact that they are single pane, have gaps at the meeting rail, and the pockets for counterweights are all things that contribute to their energy loss.  Even with restoration and high performance storms, it seems difficult to really get a significant performance bump.

Option 2: Replacing the windows entirely with with ±R-4 double pane insulated windows with low-E coatings and Argon fill. Furthermore, the new windows would be casements rather than double hung windows, improving the air infiltration numbers.

Is there any resource or study we could reference that would describe the difference in performance between restoring old wood single pane double hungs with new casement windows? In particular the improvements in infiltration would seem to be the most significant performance boost in the overall performance of a building.  Any sources and informed thoughts on this would be appreciated.