If you already have a 2310 on the SAT, I wouldn't recommend taking the ACT. Your SAT score is already high enough to qualify you for admission at any school in the country. If you take the ACT as well, you might be seen by colleges as excessively focused on standardized test scores.
To answer your second question, a 36 on the ACT and a 2400 on the SAT are viewed equally by colleges. You won't have an advantage with one versus the other. At this point, a perfect score on either test won't improve your chances by much, and I think your time would be better spent on improving the rest of your application. Work on writing a really good essay and playing up your extracurricular accomplishments instead of grasping for slightly higher numbers. Depending on what grade you're in, you might also be able to pull up your lower-than-average GPA in time for college applications.