Answer
$Ag$ has a lower 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).
Both $Ag$ and $Cu$ belong to the same IB group. From up to down, $Cu$ appears first, then comes $Ag$. Therefore, the first ionization energy would decrease from $Cu$ to $Ag$.
$Ag$ thus would have a lower ionization energy.