Answer
Slater's rules give more accurate estimate of $Z_{eff}$.
Work Step by Step
Compare the results of $Z_{eff}$ in part (a) and part (b), we find that the results produced by Slater's rules give more accurate estimate of $Z_{eff}$, since $2.2+$ is closer to $2.51+$ and $3.49+$ than $1+$.