Having tried both a while ago, I'd pick Magic Mountain everyday of the week, over Knotts. I founf Knotts very Ye Olde Fashion style theme park. Now this may appeal more to older folk, but since you have two teens, I'd think MM would be a better option. MM has a lot of thrill rides and great a set-up. Its more stylish than Knotts with its Gotham City section and other attractions. Knotts is fine, and you would enjoy the day, but I think MM a better place.
There has been a few years since we last were there, so I'm not sure what has changed.