This topic has been debated many times at both Photography on the Net (POTN) and Fred Miranda (FM) for years with no clear winner. I suggest you forget about choosing one or the other based only on IQ under ideal circumstances. You're not going to see the difference doing what you do unless you have extremely good panning skills. The prime is marginally sharper but so what? It limits what you can shoot because you can't keep changing position so you'll miss opportunities, some of which may be that "money shot". I shoot a different form of action, and based on the expectations of those viewing motorsports photos I strongly suspect the ideal photos will have slow enough shutter speeds for blurred wheels (or spokes) & background but a rider & bike that very crisp. Meeting those requirements will cancel any difference between the 2 lenses IQ wise 99% of the time.
For my needs the prime (just about ANY prime) would be useless, even if I got them free, and I rely on lenses considered to be less than ideal by most because they're superzooms, BUT they do the job, & well. They may not have the very best IQ but they do get the shot, and do it consistantly. Just remember it's not just the lens that captures the shot, it's the combination of the lens, the body, knowing how to set the variables in the body, and your ability to either pan perfectly or hold it steady enough. Blow one of those a tiny bit & the IQ is now less than what was possible when exacuted perfectly. You're not shooting stationary riders from a tripod from a fixed distance all day long so consider all the variables from your previous trips to the track before buying.
"A skill is developed through constant practice with a passion to improve, not bought."
