For what it's worth, I've owned the ATS 80mm non-HD model for about seven years. I cannot honestly say whether the HD is worth the extra cost or not. To be honest, at the time I bought it, I don't even think I was aware of the HD model (if it was truly available then). At the time, I owned a Leupold 12-40x60 and the difference was substantial to say the least. Don?t get me wrong, I like my Leupold for those backpack type hunts when weight's critical but you put the two side by side at dawn, midday or dusk and you literally shake your head.
I have taken digital pictures free hand thru the scope and admit it's a bit of a trick (take lots of pics and sort thru after words). However, my main objective is to locate and judge game at very long distances and typically I'm sitting for long periods of time, which I think the angled, eyepiece works to your necks advantage. I don't know why but it just seems more natural to have your head angled down for long periods of time. As for locating an object with the side tube it's a general area locator. Don?t bank on it being exact. Once you use it a few times it becomes fairly natural.
As for it's size, I'm always using a fairly good sized day pack for my daily deer and elk hunts out of camp. It does take up a bit of room but is manageable. When antelope hunting we're typically using a Diesel pickup to cover so much area and usually it's just as quick to jump out and open the tripod. Besides, the Diesel rattles too much to get a good view and we're always looking over dozens of goats each day. Keep in mind that the scope has the capability to rotate the body mount about it's axis so the angled eyepiece can become a bit easier to look thru mounted to a window once rotated but I would have to agree, once the rig is shut off the straight eyepiece would be more comfortable.
I guess if I had to do it from scratch I would probably lean towards the HD since all the digital camera equipment has improved so much and if I were to only limit myself to one scope knowing I was going to backpack hunt I would probably lean towards the ATS65mm.
I guess the last item I would suggest is to also get a quality tripod and head. A person could probably get away with a mid range tripod but a cheap head will be jerky and wear you down when looking for long periods of time trying to locate game you don't know is there.
You?ll know you've made the right decision when your buddy?s looking through his cheap scope and can't locate what you're looking at and ends up looking thru yours and says dammmm???..
Hope this helps. Any further questions feel free to pm me.