Rising senior here, just looking for a bit of advice.
Due to some scheduling conflicts, I wasn’t able to take my first attempt at the ACT until April of my junior year. I took it with little preparation, got a 30, re-took it, and got a 29. My superscore is currently a 31.
I decided to take the ACT because I like that there’s no penalty, and the Plan (pre-ACT) seemed significantly easier to me than the PSAT.
I’m considering taking the SAT because they offer more time per question, and I think having to rush through the ACT is part of what is keeping me from getting a good score.
My Plan score was a 28 (out of 32) and my PSAT score was a 206. My counselor said I actually did slightly better on the SAT than the ACT, although I don’t think she took into account that the Plan was out of 32 instead of 36.
The most competitive colleges I’m applying to thus far are Northeastern and Boston College, both of which have 31’s as their 25th percentile score. I would like to try for some more competitive colleges such as Stanford, Penn, and Columbia, but I’d prefer to get my scores up before doing this. And it’d also help to have a better chance at merit scholarships. 
My question is, is it worth trying to study for a completely new test (especially considering I will probably only have one shot at it), or should I continue trying to perfect my ACT score? Or should I do both - or neither?