If one is weak in Math, which is a better test to take- SAT or ACT?

The SAT and ACT are both standardized tests and hold equal importance. No one test has a higher preference for the other. Therefore, a candidate stands an equal chance of getting admission into a college based on his/her SAT or ACT score.

However, the prominent point is to choose the right exam for you. Be it SAT or ACT. There's a preconceived notion among students that the SAT is better than ACT. Notwithstanding the notion candidates have, one should focus more on self-introspection and figure out his area of interest.
Once this part of self-exploration is done you can choose which test you want to go for. Most of the job is yours to find your area of interest. If you're not sure, you can take the SAT vs. ACT diagnostic test which will help you to a greater extent. This test will help you in solving your dilemma and get rid of it.

In case specifically Math subject is your deciding factor to take up SAT or ACT, then there are other factors that need to be considered.
The ACT is time-based and demands accuracy. It is purely rhetorical.
On the other hand, the SAT is more on the analytical side and requires in-depth knowledge.
The content part in the ACT is more and shallow knowledge will work, whereas in SAT the content isn't vast hence in-depth knowledge is needed.
Conceptual clarity is required in both the tests.
Precisely, ACT has easier maths and calculators are permitted. Hence, if your concepts are clear and you just lag behind due to silly calculation mistakes then ACT is for you. To know more you can check out the Tutela blog (https://www.tutelaprep.com/blog/how-to-choose-between-sat-and-act/) on ACT vs. SAT.