Perhaps the eye test tells you Kiermaier is a better defender but the metrics don't back that up. We can compare these players metrics directly to paint a more complete picture. I'm going to utilize Varsho's last two seasons of center field play as he's produced very similar rates in these seasons, and will use Kiermaier's 2023 only as his numbers were most likely negatively affected by a bad hip in 2022.
DRS/OAA Overall Comparison
Varsho 841 innings 23 DRS (27.3 DRS/1000 innings) 18 OAA (21.4 OAA/1000 innings)
Kiermaier 981 innings 18 DRS (18.3 DRS/1000 innings) 13 OAA (13.3 OAA/1000 innings)
Kiermaier has a higher top speed compared to Varsho, but Varsho has simply been a far more effective fielder on a rate basis. It remains to be seen if Varsho can replicate these types of numbers over an entire season in center field as he's yet to play a full season there. If these numbers stick he is arguably the best center field defender in the sport.
Kiermaier also possessed a stronger throwing arm, but he hasn't turned this into extra outs compared to Varsho who has a relatively weak throwing arm in comparison. I think Varsho benefits from great technique as he gets into position very quickly, gets rid of the ball quickly (likely his years at catcher benefit him here) and he has a very accurate arm as well. It stands to reason that runners would be more likely to attempt to advance on the bases as well, giving him more opportunities to throw out runners compared to Kiermaier.
I have also included the DRS throwing component over the same timeframe. Statcast also has this type of statistic, but I did not include it as it does not break down the outs saved per position played so it's not an apples to apples comparison as Varsho split his time between left field and center field.
DRS Throwing Runs Saved
Varsho 4 DRS (4.8 DRS/1000 innings)
Kiermaier 2 DRS (2.03 DRS/1000 innings)