Abandon studying for ACT and retake SAT?

I’ve taken the SAT once, and the ACT twice.
My parents will only pay for one final test, and they are forcing me to retake the SAT in June.
I’m looking to apply to some very selective universities.

The first time I took the ACT, I received a composite score of 31. (34E, 35R, 28M, 27S)
I took it again in April, and went up one point, resulting in a composite of 32. (34E, 35R, 27M, 31S)
I took the SAT last year, and I received a only a 1230. (650 R&W, 580M)

Keep in mind that all three scores came without any sort of preparation at all.

That leaves me with around 1.5 months to study and either bring my SAT score up ~300 points, or my ACT score up 3-4 points.

Should I invest my time into preparing for the ACT or the SAT?

I would retake the ACT. Looking at some of the concordance tables, a 32 SAT equates to a 1470 and a 31 equates to a 1430. You are a long way off with a 1230 on the SAT.

The ACT is a very trainable test. Your English and writing scores are great. Science and Math are the two areas that you stand to improve the most with practice.

You should not take the SAT and really taking a test three times could raise concerns. You shouldn’t evaluate a score with some context - where you’re applying, family conditions etc… Test scores by admission committees are never evaluated on their own, always in context.

Take the ACT. Practice what you’re better at, which is the ACT.