Personally, I heavily prefer the SAT over the ACT—it's a very no-nonsense, clean way to see the capabilities of a person, and isn't relying too heavily on questions related to a specific subset of a topic.
It's definitely a better indicator of performance and capability than a GPA, in my opinion.
I am extremely familiar with both tests. What version of the SAT are you referring? The current version (re-designed a few years ago) is nearly identical to the ACT with the exception of integrating graph-reading questions into the reading and writing sections rather than having their own section.
I took them within a month of eachother, but I can't recall which I took first. I got my results back first on the SAT, so possibly it?
(On a sidenote, it's annoying that the SAT site didn't allow me to use things on it related to my test scores because of the age I got them at. Suppose it's legally mandated though.)
It's definitely a better indicator of performance and capability than a GPA, in my opinion.