The SAT is in no way more "rigorous" than the ACT. The theoretical difference between ACT and SAT is a slight shift between "achievement" (ACT) vs "potential" (SAT) although the reality is that this is pretty minimal. That is to say, ACT was supposed to slightly more reflect what you had learned in HS, while SAT was slightly closer to something like an IQ test that changed less by HS education.

In practice, the only difference between ACT and SAT for decades has been geographic preference. East&West coast schools primarily use SAT and Midwest/Central schools primarily use ACT.