Matt,

1. Higher STC will reduce the noise that is coming through the window assemblies. If that is the dominant source of noise, the difference will be significant and especially with something in the 40+ range like the Milgard product. Depending on your wall construction, your walls will be the weak point in the assembly as will the ceiling/attic assembly.

2. They do work and are well constructed. They come as a pretty penny so be prepared for that number as well. When you are talking about the Milgard windows, just go for the top end. You are already spending a good chunk of money on the window to begin with and the install. The delta between the three levels of STC is not that significant if I recall correctly.

Good luck.

thanks a lot for the advice. I've also heard that these quietline windows are very thick (a couple of inches thicker than standard 2-pane windows) and so might require some extra molding or ledges to be installed around the window opening for aesthetic reasons. Do you have any thoughts on this? Thanks

Yes. They are deeper than standard window jambs.

If your wall thickness allows for it, you can move them further in if you can or they can be capped to outside to create the proper drainage.