The biggest difference is the 13 received and wet sump engine where as the 12 has a Dry sump engine.Don't know why Polaris decided to do this other than it was cheaper for them to make the wet sump and charge more for it. You'll get 500 different opinions on which is better but if you ask any engine builder what they want in their Race set up they will most likely tell you Dry sump. Other than that the colors are different and there is a Walker Evens edition with the WE shocks instead of the Fox shocks. Also the 12's received a steering geometry change over the 2011 which some people felt was much needed. The 13 gets the same steering change along with optional EPS on both the 12 and 13 LE models.
I just bought a 12 but almost bought a 13 but decided on the 12 because of the dry sump set up.