I'm going with:
Splat1 Multiplayer > Splat2 Multiplayer
Splat2 Singleplayer > Splat1 Singleplayer
Which seems to be the general consensus of the thread.
To me, I think:
Splat1 Multiplayer > Splat2 Multiplayer (or with update music included, ">>>")
Splat2 Singleplayer = Splat1 Singleplayer
Well, technically they only brought back 3 songs (Tentacular Circus, Cephaloparade and Octoling Rendezvous) which I wouldn't really count as "reusing much of the music", but I suppose it depends on how you view it.
Really, only three? Well, they kept all the most memorable music to me, which is why I guess I thought there more reused songs. A couple of the new songs are remixes though: Octo Eight-Step is a remix of Eight-Legged Advance, although I prefer Eight-Legged Advance. The boss music of Splatoon 2 is sort of a remix of the first game's boss music, I think? I don't know which one I prefer though; the new boss music is more upbeat and just sounds more like boss music, but can get pretty annoying for some of the longer bosses. Spicy Calamari Inkantation is obviously a remix of Calamari Inkantation, and while neither are bad, I easily prefer the original (I'm also biased, lol).There are likely more remixes, but I probably haven't played Splatoon 1's single player since early 2016... I
really wished they kept the mission clear theme though, along with the old animation where you pat the Zapfish on its head. It felt really accomplishing...