**Take a couple more ACT [ACT practice tests tests][1] and see if you can improve your ACT score even more.** Right now your [your scores are about even, even with each other][2], but I agree that your SAT reading Reading score is somewhat of a concern if you're applying to the most selective colleges (which I assume is the case if you're still looking for a higher score).
**If you can get your ACT score in the 35-36 range after a little more prep, I would go ahead and take it instead of retaking the SAT.** If you can get a 33 on the ACT without prep and you've struggled with raising your SAT score from where it is now, it stands to reason that the ACT is a better fit for your skills.
[1]: http://blog.prepscholar.com/printable-act-practice-tests-5-free
[2]: http://blog.prepscholar.com/act-to-sat-conversion