Answer
Mark McGwire feat was more impressive than Roger Maris'.
Work Step by Step
Mark McGwire
$z_{MM}=\frac{y-\mu}{\rho}=\frac{70-20.7}{12.74}=3.87$
Roger Maris
$z_{RM}=\frac{y-\mu}{\rho}=\frac{61-18.8}{13.37}=3.16$
As $z_{MM}>z_{RM}$, Mark McGwire feat was more impressive than Roger Maris'.
