Feb. 7, 2021
If you wait until December to take your SAT, you can still get those scores to colleges before your regular-decision verdicts are finalized. My advice would be to wait until you see the December SAT scores before you decide whether or not to send them out. If you are happy with the scores, you can immediately order score reports sent to your colleges, and you can also ask your guidance counselor to email or fax the scores to colleges in the meantime. Although the colleges will still require “official" scores from the College Board, they will most likely be willing to consider the scores that come directly from your high school while they wait for the “real" ones. And if these real ones show up a bit after the Regular Decision deadline, the vast majority of colleges will consider them anyway.
Thus, if you're truly delighted with your December SAT, that's how you should proceed. And if your December SAT results are not comparable to your ACT score, then don't send them out at all. (Of course, if you've been admitted to an Early Decision college, you also have no need to send out your SAT ... unless it's a college that gives merit aid, and your SAT score seems strong enough to boost your merit-scholarship chances.)
But one thing that's confusing to me is this ... as a rising senior, you have most likely already taken the SAT or the ACT or maybe both. Thus, if I were your college counselor, I would advise you to plan your senior testing schedule around the results you've received so far. For instance, if you did better on the ACT than on the SAT, then you might want to focus only on the ACT for the fall re-take ... or vice versa. If both tests came out roughly the same, you might consider re-taking only the one where you feel that you can make the most improvement. (For many students, that would be the ACT because the strict time limits tend to hurt students on the first try, but then—after a little practice and experience—those time limits don't feel as restrictive.)
So ... depending on which tests you've already taken and how well you did on them, I might recommend a different schedule than your current one. It would be helpful to know more about your testing history as well as your testing plans.
I long for the era when I went to high school. Back then, in the 1960's, students typically took either the SAT or the ACT (whichever was most popular in their geographic region) but rarely both. And most of us took the test no more than once or twice (commonly spring of junior year and/or fall of senior year and possibly one of each). Now there are too many options and way too much testing, so I can't blame you for being anxious. Sadly, it comes with the testing territory these days.