SCS selects it students from the national/international pool while EECS pulls most its students from instate. SCS accepts less than 20 ABC boys every year. While CA is competitive, but most tops ABC boys, including 80% top math/stem talents are from outside of California. Beside MS, SCS is these kid's top choice. This makes SCS extremly hard to get in for ABC boys. A CA ABC boy will have to match the calibers of these Non-CA kids to get in to SCS, which is likely to be significantly harder. Wether a CA boy prefers to go EECS since it makes more economic sense is a different issue.
SCS likes top Math performers and requirea all students to submit AMC and AIME scores if they have them. MIT and CalTech are the only other schools requiring these scores. A few years back it was said that SCS students had a medium score of 6. That was about top 20% of AIME scorers. Today it is only going to be higher for ABC boys.
That said, SCS can be easier for CA girls since EECS does not have gender AA but SCS has.