In general, there are a lot of misconceptions about how the two tests are perceived in a side-by-side comparison, but ultimately both tests carry equal weight. All four-year institutions accept both tests and use a conversion system to compare applicants. That said, every institution in the country gives special consideration to PSAT National Merit, which a student can only achieve through taking both the PSAT and SAT.
If you really want a competitive edge, take both tests.