for a new driver, here's the best way to explain WHY you want one over the other.

17.5 - easiest to drive because you use full range of motion with trigger.
13.5 - next easiest to drive because you use full range of motion with trigger, just go a bit faster
8.5 - uses less trigger range of motion because of the amount of power. If setup right, you will usually use about half of your trigger. Fast guys will be able to work in more than slower guys. Harder to drive when you have half the throttle range as the 17.5 and twice the power.

All in all, if you were only getting 1 motor, I would say get a 13.5 and add boost until you can make it around the track and be less than 1 second off the fast guys per lap. After that, move up to a 9.5. You will have more power than you know what to do with. I ran an 8.5 and wished I had bought the 9.5 instead because I would have been able to control it better.