Yeah Mike you got it right. I have used both versions as you stated and my preference is the same as yours.
Mark, in this case I used the CSC version only because I didn't have the Google Earth version loaded on this pc and it was quicker to use the CSC (both actually use Google Earth so you need to download that first for either to work).
What I did (after downloading Google Earth),
1. go to Clear Sky Chart home page www.cleardarksky.com/csk
2. "Browse" for locations in the State of Washington
3. Choose a location of interest (closest to the site of interest) and follow that link - in this case "Kennewick-Pasco-Richland"
4. In the "Nifty Links" area of the location you've chosen you'll see "light pollution map" and select that link
5. In small print, just above the LP map for the area you have chosen, you see a link labled: "Get this map as a Google Earth image overlay" Click that baby and it'll launch Google Earth with your LP overlay (after following the "open" and "allow" buttons). Shazam!