While I cannot answer your question, I did compare the 65 vs. 80 HD inside a Cabelas store. The 80 HD gathered 30% more light in my opinion. Since I like to stay out until the last possible minute of light, I bit the bullet (weight wise) and went with the 80 HD. The 80HD was recommended by Coues Sniper here as well, and he's 100% correct.
My range is as follows: First 100 Yds is in the open, the last 250 yds is a 20' wide path cut thru 40' to 60' tall Oak and Maple trees. With the 80HD, I can see .30 cal bullets in paper @ 300 Yds in the shade on matt paper.
I didn't do a HD vs. non HD comparison because I lean towards the "buy once" theory. (It's taken a few years and many wasted $$ to learn it.)