Hello! I am a freshman in high school, and I am running mid/long distance track this year. Since my birthday just came, I was given the option between under armor kick distance spikes and nike zoom rival m 8. I could run the 800m, 1600m, and possibly the 2 mile. I won't be running all of these events, but I will be running two of them. Which ones is up to my coach. Which is the better shoe for these events and why? Also, the kick distance says stiffness Index 8.0 on the bottom. What does that mean?