If you’re not picky about counting inches or trying to age a sheep when glassing big open country go with 15’s. They’re easier on the eyes to sit behind all day using both eyes.
If you’re scanning within about 2 miles with your 8/10’s already I’d lean to a spotter to count inches and get a really detailed look
Yea I hunt big country in Idaho, Colorado, Nevada and Utah. Haven’t really needed much more but the 15s and a spotter would be nice every once in a while