They are both amazing.
I must admit I felt like I was cheating on Disneyland when I walked towards the California Adventure gates (directly opposite Disney). CA opened in 2001 on what was the old car park for the past 40 years. Unfortunately it did not do nearly as well as had been projected and they kept adding new rides and sections, increasing popularity but it was when Cars Land opening last year that has really seen numbers soar.
Cars land is absolutely amazing, it is exactly like you are on a film set which is hard to get your head around when it is an animated film. The Disney attention to detail is mind blowing. It is something you need to experience both during the day and then again lit up at night. Mater and Lightning McQueen drive down the street during the day and I feel a little embarrassed at how excited I got seeing them.
Radiator Springs Racers is the hottest ride now and rightly so, it is so much fun. The lines are painfully long, we waited 2 hours the first time. In a separate blog post I will talk all about the FAST PASS and how you must master it.