I played ARMS on my nephew's Switch and I owned a Wii U so I definitely owned Splatoon, pretty much playing a good amount of time on it.
If I had to choose on getting ARMS or Splatoon 2, I'd choose Splatoon 2. ARMS can be fun but something about it just didn't capture the joyous feeling of playing Splatoon. ARMS controls definitely pale in comparison to Splatoon's controls on the Wii U. The same can be said for the music, while ARMS' music is alright, Splatoon's music was more memorable. I feel like ARMS comes across as more of a one trick pony than Splatoon.
Maybe I'll like ARMS more after several updates but from playing the game, I'd wait until it goes on sale and buy Splatoon 2 on release. Only reason I don't play the first one so much because most my friends moved on from the game and also the end of the Splatfests.
Personally I didn't like Arms local MP (which would have been my main mode of play in that) because it was messy and hard to follow so I sold that. Splatoon doesn't have split screen so I'm not buying that.