I just got back from my trip and visited both Knotts and Six Flags Magic Mountain. If you're wanting a Roller coaster day, SFMM is tough to beat. They have most coasters as well as several one of a kind (at least in the U.S.) coaster. The only thing is if you go on a busy day, be prepared to either wait in long lines or buy a "flash pass". You may want to check their calendar because certain times of the year they are only open on weekends. I went to SMFF last Sunday and the lines were very manageable.
Knotts is much closer and I think a more "well rounded park". Knotts has a couple of dark rides, and a few shows that run year round, a train, etc. They also had better food options.
If you do go to SFMM you will probably want a nights hotel in the area. It could take an hour and a half to get there depending on the traffic.