Best way to visit Six Flags Magic Mountain and Knott's Berry Farm? Transportation

Hi everyone, I'll be visiting LA/Long Beach in March for a conference so looking for some advice about transport. I'm flying in and out of LAX due to flight options. I was looking at renting at car near LAX, then driving to Valencia to do Six Flags Magic Mountain and then head to Long Beach to do Knott's Berry Farm and attend my conference/return car. Is there an option I'm not thinking of or is renting a car the best to do these two spots?