I prefer 15" wheels for street use, but 17"s for the track as we have a lot of tire choices.
I also like the looks of tire filling up the wheel wells, rather than wheels. The latter makes Cobra's look like a street rod, not a old school muscle car. There's nothing wrong with either though. Plus, less sidewall on the street is harsh, but less sidewall on the road race track is much better. Too bad there isn't more tires for the 15", as time goes by, there's less and less.
