Not necessarily. The C fund may outperform if you are a buy and holder, but since my system is based on trading for shorter periods, I prefer the volatility of the S fund pretty much all the time.
One thing you could look at is, when my system gets it's next buy signal, check to see which fund has dropped the farthest from my previous sell signal. This "may" indicate that it'll bounce back up higher. This will normally just be a choice between the S and I funds though. If you don't mind the I fund, it may be a better choice. I'll stick to the S fund though.....