As with most things, some people will say the 2015 Pro V's are much better than the older 2013 version and some will say the 2013 version is better.  Best advice, get a sleeve of both and compare them for yourself on the course to see which works best for you.  Regardless of the choice you make, you will eventually need to switch to the 2015 Pro V. 

I'm not exactly the best golfer, so I would tend to notice less of a difference, but this is my experience.  There is less difference between the 2011 and 2013 than the 2013 to 2015.

2013 brought a more durable cover, that's for sure.  I notice far less scuffs on the ball than the 2011, but overall the performance, spin, and flight characteristics were very similar for me.  2015, on shorter iron shots I notice more spin.  I see a little more "bend" to my 8, 9, and PW right or left depending on my swing.  

All are great, though and my ball of choice. I'm still buying 2013 where I can find them, however, if only to save a few bucks.