I've been waffling between 10% and 15% for years. Most recently I bough a case of 15% as it was only $2/gallon more than the 10%. The only advantage I have noticed in my sport flying is that I seem to be able to get a more consistant low idle with the 15%. That could just as easily be due to conditions of the day and circumstance. Possibly a higher top-end, but I seldom fly full throttle anyway.
I can tell you fresh 10% does better than stale 15%. ;-)