That really depends. I used to buy Racecar Engineering every month just to keep up w/ whatever's happening in motorsport. I picked up Race Tech Magazine whenever there was something really good on, as it is harder to find around here.
However, if you look at Race Tech's website, you'll see they have a lot of other publications that may suit your interests. For example, their engine magazine looks really good, and now they have an aerodynamicists' publication.
http://www.racetechmag.com/
I can't really help you as to which one is better as I stopped reading both. But in my experience I would probably subscribe to Race Tech.