Answer
$Cl$ has smaller first ionization energy.
Work Step by Step
The periodic trend of the first ionization energy:
- Going from left to right across a period, the first ionization energy increases (since effective nuclear charge increases and atomic radii decrease).
- Going from up to down in a group, the first ionization energy decreases (since effective nuclear charge only increases slightly but atomic radii increase largely).
$Cl$ and $Ar$ both belong to the same period. From left to right, $Cl$ appears first, then comes $Ar$. Therefore, the first ionization energy would increase from $Cl$ to $Ar$.
So $Cl$ has smaller first ionization energy.