Should I Retake the SATs or ACTs

So, since the deadline to register for the Dec. ACT and SAT are at the end of this week, and I need to decide to which one to sign up for. Even though I qualify for fee waivers, I don’t feel like taking both; would be too much stress since the tests are one week apart from each other. Anyways, I need some help in making a decision. I’ve taken the SATs 3 times already, my score improved drastically between the first the second test (1610-1950), but went down after my 3rd try (1950-1830). As for the ACTs, I’ve only just taken it in October, so I’m not sure if I did well on it or not. So what do you guys think, which should I register for? (It’s my last chance before I have to submit applications to college, so I can’t just sign up for a later date).

P.S: I kinda need an answer soon, since like I said before, registration deadlines are in like 2 days.