This is a highly personal choice, but I’ll give you my opinion. There is always going to be some hill that you can’t glass effectively, even with a spotter.... that said, for glassing within your effective hunting range, 15 power binos will definitely help and I think there is an argument for owning 10’s, 15’s and a spotter. I run all my glass off a tripod with a panhead and am a pretty strong believer in that system.
I haven’t tried everything, but I have to say, I think the 15x56 Swarovski’s are the finest piece of glass to spend a day looking through. They really are a pleasure to glass with and I haven’t ever thought twice about the purchase. I also have a Kowa 25-60 spotter that I love, but I really only use that to determine legality, to better evaluate an animal that I am considering stalking, or to pick apart patches of shade. The vast majority of my day is spent behind my 10’s and 15’s though. In my opinion spotters are not very pleasant to glass through.
As far as weight goes.... again, a highly personal decision. How much is too much for you? Depending on location, when I hunt alone I usually will leave the spotter at home. But when I pack out a deer I do it in one trip and an extra 3.5 pounds does make a difference. Also in San Diego I’m not over-analyzing rack size. The 15’s will at the very least give me an idea of the deer’s frame and I can make a decision based on that. If you are counting inches on a big buck in Idaho that spotter might make all the difference. But if that’s the case I would argue that you need a bigger spotter anyway, but it all depends on the distance.
If it isn’t clear already my 15’s are my favorite glass to use if any distance is involved. As for your question about 20x70.... the issue with that is that much of the weight from glassing equipment comes from the objective lense (the big one that faces the animal), so having two 70 mm objectives makes a binocular that night weigh 5 or 6 pounds. You are basically asking about Kowa big eyes, which you can buy.
