TH3-D0S3R on 12 May 2018
As someone who put well over 200 hours into both versions, I would say Splatoon 2 easily. The mechanics are similar enough, so at that point it comes down to which game has more content. Splatoon 2 had as much content as Splatoon did at launch (minus maps), plus a hoard mode and more fleshed out single player.
