12-6 is a little faster top speed, takes slightly longer to reach top speed. 13-5 has more pull; shorter take-offs, better vertical performance, a touch slower top speed but better acceleration.

Trainers tend to be 'dirty' designs, meaning they have lots of drag (largely due to the fat, flat bottomed airfoil). They also are not the most graceful aerobats, so I would go with the 12-6, but it's probably best to try them both and see what YOU personally like.