I don't have 15x binos, but the difference between 10x alpha quality glass and the mid range can be extreme at least with my eyes. I had the good Minox 13x56 binos that I read a lot of great reviews on and traded them in on the 10x42 Zeiss Victory binos I have now. I guarantee that I can see more detail on deer tines with the 10x Zeiss in any light than with the 13x Minox that are still darn good binos. I was impressed with the 13x until I got the Zeiss. I'm cheap but sometimes things are worth the extra money to buy once and enjoy.
My advice would be to find a place to try 15x Swaro binos, I have been wanting a pair myself after looking through an outfitters on a muley hunt a few years back. The guide he had working for him had 10-15 Leica Duovids that were also impressive although not quite up to the Swaros on 15x. I'd be interested in seeing the Nikon ones also but will have to see them to become a believer. The Vortex Kaibabs are awful nice from the short time I looked through a pair also. If you get the chance take the Swaros, Vortex, and Nikons outdoors and try them on small detailed things at decent range. In a store with bright lights everything looks better than it is, try picking out individual leaves at 600yds outdoors for example. The fine crisp detail is what you are looking for to identify and evaluate animals. If you get the chance please do a post on your thoughts also.
I guess my point is to not get overly hung up on magnification. Top quality glass with a lower magnification will beat mid-range glass with higher magnification for my eyes at least. Go somewhere to try them outdoors and see what fits you the best evaluating fine detail at range.