Well, the decision between a M or SA helmet hinges upon what you see yourself doing. For SCCA or NASA autocross, and most HPDE events a M rated helmet will be just fine. If you're going to go wheel to wheel racing, then the SA is the way to go.

The way I was thinking about it when I bought my helmet was what I saw myself doing in about 5 - 10 years. Eventually, I want to race with NASA. So, the SA was what I went with. That way I don't have to buy another helmet once I have a real racecar.