Absolutely agree with the bold. In OP's case though, there's some pretty major disparity between the ACT (19 in Math vs. 29/30 in english/reading) sections while there is little disparity in the SAT sections (only 40 points). That tells me OP probably doesn't have a major problem with the strategy of test taking, but rather with the material which is harder on ACT math.
ACT science is just critical thinking, so with some very basic help with taking the right steps in logic and some practice tests, I think OP could bump that up to a 25-26 pretty easily. The math is tougher, but I would suggest seeing a tutor and making sure that the basic stuff is solid through geometry. Strong enough so OP is only missing 1 or maybe 2 questions on material of that level and easier. Then focus on trying to improve on the hard stuff like trig and pre-calc. The other thing is that math can be the toughest or easiest to guess on. Either the answers will all be completely random numbers and one must simply do it right to come to the answer or the options will be related (ex. -3, -1, 0, 1, 3) and OP can narrow it down pretty easily. Since OP only got a 19 there, I would think that he probably guessed on a good number of them, so the ACT would probably favor that since he won't lose points for guessing wrong.
I agree that he should pick one test and just focus on that one. I also think the ACT is the better option here as there seems to be more room for improvement in the math and science sections, but that's just what I would do.