Which test should I study for? ACT/SAT or both?

First of all, I can not afford a course or anything fancy. The most I am planning on getting is blue book, act book, etc. I took both tests once without studying. On the SAT I got a 1710 ( CR 630, M 590, W 490). On the ACT I got a 28 ( E 24, M 29, R 32, S 27). I am planning on taking both tests in October. Should I improve both test scores or focus on one?

If you can afford the internet, there are tons of resources. If I was you, I would not depend on preliminary results. Taking one test for each then trying to determine which one is the best seems pointless.