It seems like an unpopular opinion (just look at the comments above), but I would go with ARMS, and here are the reasons why:
* it's awesome in multiplayer, both online and splitscreen. And don't be fooled into thinking that it does not have active player base, since it is still played big time
* it utilizes motion controls in great way and is fun to play with that way (Splatoon 2 has motion aiming, which is also great, but still inferior - all the other games with aiming has this as well in general) - and once you need precision, you can easily switch to gamepad style of playing
* it's a new and fresh type of a fighting game, and if you give it a shot without judging it by the comparison to traditional ones, it is really deep, easy to learn and hard to master experience
Me and my friends play it together every week or so, and since I am the only one owning Switch, we cannot do the same with Splatoon 2 (no splitscreen). And also, playing with friends is kind of broken (me and my other friend played it many times and we always ended up in different teams), and if you have limited time, there is no way to choose your favorite ranked game mode. So, this settled it for me.