SAT or ACT?!?

<p>So I recently took the ACT last Saturday, and I felt like I did horrible. I did so bad, I would be surprised if I got a score higher than 25. I prepared for the test by taking 4 practice tests under a timer (got higher than 30 for all of them), yet, when I was actually in front of the test, I lost everything. This never happens to me, but, for some reason, it did. Maybe it was just one of those days, or maybe I didn't prepare enough - particularly on the science section.
Now I have an SAT test this Saturday, and I have one week to study. I got a 1950 on my first try in the spring, and I want, at least, a score higher than 2100. I know it's probably not possible now, because I only have one week, so I plan to sign up for the December SAT. My question is, should I also sign up for the December ACT? I have a feeling if I work more on my time management, I can "redeem" myself and get a good score. However, that would mean I have to study for SAT and ACT at the same time.
Should I just abandon the ACT and study for the SAT? I'm predicting a score above 2000 this Saturday, and, if I take enough practice tests and study, a 2100+ in December. However, if I study for both tests, a 2100 doesn't seem possible.... What should I do?</p>