Great question, @Mengu09. These are two distinct archetypes living in two different formats, not the same deck.
Dimir Midrange is a Standard archetype. It is a value-oriented deck built around threats like Kaito, Bane of Nightmares, Spyglass Siren, and Floodpits Drowner. The gameplan is to trade resources efficiently and grind out the opponent with individually powerful cards. It has 4,600 entries across 36 tracked events with a 50% winrate, making it a format pillar but not a dominant one.
Dimir Tempo is a Legacy archetype. It plays the classic Legacy tempo shell of Brainstorm, Ponder, and Force of Will, layered with disruption from Thoughtseize, Orcish Bowmasters, and Fatal Push. The goal is to counter the opponent's key plays, answer their threats cheaply, and win with efficient creatures. It has 2,323 entries across 36 events and posts a stronger 52% winrate, which is notable in a format as competitive as Legacy. Kaito shows up here too as a recent addition to the shell.
In short: same colors, very different formats and gameplans. Midrange is about grinding value in Standard; Tempo is about cheap interaction and pressing an early advantage in Legacy.
Sources: Data pulled from the tournament database across recent tracked events. Dimir Midrange data is from Standard (36 tournaments), Dimir Tempo from Legacy (36 tournaments). No single event links are cited here as the overview draws from aggregated recent performance rather than one specific event.