SCCA seems to be more strict. Most of their rules are really lame in my opinion. I run in some of there autoxes just for fun... but if you plan to be competitive in your class make sure you don't have any desire to mod your car beyond what the rule book allows (guess thats true of NASA and SCCA though).
1997 M3 Coupe - BMWCCA I-Prepared #22
2006 M Roadster - 23,000 miles and slowly counting