I'm guessing you are here because in your household the discussion on whether to restore or replace your original windows has been raging for a while.
Please keep in mind that I am biased and consequently the information you will find here is biased also. I favor restoration over replacement. When people ask me why my reply is: why would you replace a product that has proven itself for decades, possibly a century or more, and replace it with something that has a limited lifetime warranty which may boil down to 5 - 10 years, or less if the manufacturer goes out of business.
Now for that biased information:
Keeping what you have is by far the greenest option. Of the triad "Reduce, Reuse, Recycle" not throwing away your old windows is at the forefront of the options. It's the "Reduce" option.
If you have concerns about energy loss or sound infiltration, there are other, better, and often cheaper (in the long run) options available to you, rather than replacement. See also under "Why Restore?/Don't just take my word for it..."
In the other sub-categories of this page you will find general discussions of a variety of topics. I hope they are of help in your decision-making process.
Reduce, Reuse, Recycle.