Are you interested in just spotting deer or are you interested in spotting plus field judging bucks? If field judging isn't important to you I would buy 15x binos and scratch the spotter idea. If you are like me and trophy judging is super important I would go with 8 or 10x binos plus a spotter. It may be frustrating spotting a buck with 15x binos across a canyon and not being able to get an estimate of score, You can likely tell if it's 170 or 180 class buck with a spotter? If that doesn't bother you then go for 15x!
You will find out in a hurry that it is next to impossible to field judge game with 15x unless you are super close. You'll also find out that you can spot an antler, ear, or rump of a buck in a small patch of trees with a spotter that you won't be able to find with 15x binos. I am super confident that I can find more game in big canyon country with my 10x binos plus Swaro scope than anyone with 15x binos! It may be tedious but I can find game that are impossible to find and field judge with 15x.
With that said, I'd love to have a pair of 10x, 15x, plus Swaro spotter on a hunt! Guys are right that 15x would be nice to spot game in vast, open areas. I haven't gotten to the point where I want to swap back and forth between 15x and a spotter on a tripod....which likely would work well together. I'm usually trying to figure out strategies to loose weight in my pack so I'm not too terribly interested in 15x binos!