It really is up to preference. Both are different genres, so it depends on preference. Splatoon is single player, while smash has 8 players that you can play with. Splatoon is possibly the best shooter this gen, while smash bros is the best fighting game this generation, I prefer them over call of duty black ops 3 or mortal combat x any day.
Personally, I like Splatoon more, just because of how unique it is and the cool integration of the gamepad, but smash is not far from it. Personally, go what is best for you.