I just did practise tests for the SAT and ACT, getting around 1300-1350 on SAT, and 27-28 on ACT which is pretty much the same. I’m in my snr year so I can only pick one to do. Which is easier to improve on?
In my opinion, the ACT is easier to improve on because the topics it has are usually things you’ve already learned in the years prior high school and the only really hard thing about it is timing. All you would have to practice is being better with test taking strategies and the timing of each section so that you have enough time to do all the problems and not guess on any.
But if it’s your senior year, why are you still taking SATs and ACTs? Most colleges don’t accept anything past the December test, I’m pretty sure.
Our senior year just started, we start in Feb and end in Dec so I’ll be applying this year 
Think about which test you felt more comfortable taking and which test you would prefer to focus on. Did you feel like you were more rushed for time during one test more than the other? Also think about your strengths. The ACT has the science section while the SAT does not. The SAT has a no calculator math section while the ACT does not. Hope that helps!