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).
1995 M3 Coupe - HPDE/Track Prepped - For Sale
1997 M3 Coupe - BMWCCA I-Prepared Race Car
2011 335d - Das Uber Commuter (35.1MPG Avg)