For what it's worth I started racing Touring cars about a year ago. I have raced off-road for many years. I started racing 13.5 and 17.5 on a 140 x 60 asphalt track. I stopped racing 13.5 for two reasons. I broke a few more parts than 17.5 and I was getting lapped by our local pro's. I did not feel like I was racing as I always had to move out of the way. Keep in mind that I may have finished 5th out of 8 guys. I then changed to 17.5 and 21.5 TC. I now run 21.5 TC because I want to concentrate on one car via set up etc. And feel that it is slightly easier to concentrate on my lines. I do ok with 17.5 when I race. My skill level is pretty good, but we have skilled racers...some travel from far away to race at our facility...
The bottom line is that I enjoy 21.5 TC the most.