Smog.
Headers I feel are way easier to hide and sneak by an inspection. SC only has 2 options: Remove and replace for inspections, or, find a smog guy that will overlook the inspection and pass on emissions.
Trust me, I'd rather have the ESS now that its been a tested and proven kit. Maybe I'll plan it so I get one installed right after an inspection and worry about smog 2 yrs after that