Answer
The SAT's z-score is higher, therefore it is relatively better.
Work Step by Step
The z-scores: SAT: $z=\frac{value-mean}{standard \ deviation}=\frac{1490-1518}{325}=-0.086$.
ACT: $z=\frac{value-mean}{standard \ deviation}=\frac{17-21.1}{4.8}=-0.854$. The SAT's z-score is higher, therefore it is relatively better.