Here are my thoughts on the subject of a spotter vs 15's. I asked myself this same question when I started researching spotting scopes about 7 years ago with the intent to get a spotter. After researching and reading some, I went with 15's, and still haven't bought a spotter. I mainly hunt in AZ, so 15's are very popular and widely used down here. I typically don't 'trophy' hunt, and just use the 15's to find animals. However, sitting on a hill and glassing all the surrounding country usually allows me to glass up the best deer or elk around for the area.
My setup is 8x binos that I carry around my neck (with a tripod adapter) and a set of swaro 15's. I carry a (sturdy) tripod to use both pair of binos off of.
My first hunt of the year this year was a pack in archery hunt in CO. I left the 15's at home and only brought my 8's and a lightweight tripod. Only comment about this is I was glassing up mule deer easily with the 8's, but some of the ones at longer distances (>1 mile) I was having a hard time judging them, and depending on angles, light, etc, I sometimes felt a little underpowered with my 8's compared to my buddies 10x. Then again, after comparing them, I had a hard time noticing the 2x difference. (I won't go into that any more, as that is a whole nother topic...) There are a few times I wish I had a compact spotter to have a better look at a buck, but at the same time, we weren't being picky and so just finding deer and seeing antlers was good enough.
For other deer and elk hunts (here in AZ), I use the 15's to find animals. For my wife's limited opportunity elk hunt here in AZ we were glassing up elk at 3 miles away with 15's. Antler detail didn't really matter since she had a cow tag. Just finding animals was the important thing. On my southern AZ coues hunt a few weeks ago, we used the 15's to find the deer. We'd glass up coues bucks a mile away, and while yeah, I wished I had a spotter to really look at the antlers, we could tell through the 15's whether the deer was a buck and if the buck was a shooter or not. We weren't trophy hunting, but we were passing up spikes and (young) forkies too. My buddies with 10's could find the deer, but couldn't always put antlers on them.
I recently helped out a buddy on his AZ Strip mule deer hunt. This was a trophy hunt. For that hunt, we did a lot of glassing, me with my 15's (and sometimes with 8's depending on distance) and him with 10's and a swaro 80mm spotter. I got to try out the spotter and he tried out my 15's on that hunt. The spotter was nice, but I personally have a hard time using only one eye to look through a spotter. I only used the spotter a few times to glass by looking under trees at long distances midday, but it was rough. I'm sure with more practice it'd be easier, but I felt like I could see more and better with my 15's, even if it was lacking the higher zoom. The few times I found something questionable with the 15's and we put the spotter on it to figure out if it was a tree/rock or a deer, or pulled out the spotter to evaluate the buck, the spotter was helpful, but not as good as I thought it would be. Zoom in too far too look in the shade of a tree, and it is still dark. The spotter also shook more than I wanted on my buddy's tripod setup (need a good solid tripod head for a high power spotter [=more weight]). I've tried cheap spotters before, and was never happy. I thought I'd be happier with the swaro 80mm, but I still think I expected too much. Heat waves also were a concern with the spotter. They were sometimes manageable with the 15's but cranking up the power on the spotter made what we were looking at look worse.
However, in saying that, for a couple of sheep hunts I've helped on, a spotter has been instrumental in judging sheep, or even glassing at 2-3 miles out. However, for overall glassing, 15's are hard to beat and were used to find the sheep.
For me, for the animals I hunt, and the way I hunt them, I carry 8's around my neck for glassing while on the move or on a tripod for less than 600 yards, and 15's on a tripod (carried in the pack) for glassing at greater than 600 yards. I still want a spotter, but it is low on my priority list and I know I'll mainly use it to evaluate animals and use it the least of all my optics. I'd also like to get into digiscoping with a spotter. I'm continually trying to cut weight out of my pack, so the thought of adding a big (heavy) spotter isn't appealing. I don't think a small 50mm (13-40 power) would add any benefit to my 15's, since I think I'd lose light gathering ability and only be able to use one eye. A small spotter in conjunction with my 8's would work well, I believe. I think the biggest benefit is putting a pair of binos on a tripod. For 8's and 10's, you don't need a very big/sturdy tripod either, unless you want to stand up a lot.
Sorry for the wordy explanation...
