Who has baseball's best bullpen: the Yankees or the Brewers?

Entering the 2018 season, no bullpen was hyped as much as the Yankees' relief corps. The late-inning crew in the Bronx earned the vaunted "superbullpen" label, with multiple closer-capable arms headlined by the fireballing Aroldis Chapman. By and large, the Yankees bullpen has not disappointed: They rank 7th in MLB in bullpen ERA, and 1st in K/9. However, two months into the season, another bullpen has emerged as being similarly worthy of the "superbullpen" moniker -- the Brewers'.
The Brewers relievers rank 1st in ERA and 2nd in K/9. How are they doing it? They've built a bullpen that looks a lot more like the famous Yankees group than one might realize.
But whose is better?