I took a quick count and out of 24 PCA sprint races this year, the 1st place finishes were distributed as follows:
11 - 911 SC (prepared)
4 - 944 Turbo (86-88)
3 - 944S2
2 - 964 C4
1 - 911 3.2
1 - 968
1 - '70 911 (prepared)
1 - '75 911 (prepared)
While the numbers suggest that a Prepared SC is the way to go in F Class, it's difficult to draw such a conclusion since not all of these cars battled head to head at every event. Also, there are simply more 951's, S2's and SC's in Club Racing so the odds of nabbing the top spot are definitely in their favor.
Food for thought.