Is to take a FREE diagnostic test of both the SAT and the ACT, and then compare your scores. That way, you can see if there is a clear winner and one test starts you off with a relatively higher baseline to work off of, or if there isn't, you can flip a coin, and choose which one to study for. 

Colleges view the SAT and the ACT completely and utterly the same. There really is no test bias, and so the best thing you can do before spending your time studying for the test that that is not best for you, is to determine which test is worthy of your time in preparation and $$ if you are signing up for a course. 

There are only 7 official practice tests for the Redesigned SAT, provided by the College Board, the makers of the test. These are the best tests to use when seeing if the new SAT is the right test for you. Choose one of the seven tests, print out each section, and take the test sequentially and continuously without distraction and with a timer, as if under real testing conditions. Then, grab one of the ACT practice tests available for free online (make sure it is official and not a prep from a test prep company), and do the same. 

Once you have both of your test scores in hand from your practice diagnostic tests, compare and contrast. Are there noticeable differences? If so, choose the one that starts you off a little higher! You can feel better proceeding to study beginning from a higher baseline.