The experience in our family – which a friend who runs a test-taking outfit has confirmed to me – is that many students will do better on either the SAT or the ACT.
Having written that, a 1550 SAT is quite good; according to the concordance tables, that is equivalent to a 35 ACT. So unless your daughter can get a 36 ACT, I don’t know that there would be much advantage to taking the ACT because there is not a lot of potential upside.