It looks like choosing between Naughty and Nice is about which mission you complete first, meaning you literally can't play certain game modes until you complete the choice or else you'll get locked onto the path you don't want.
kattzkitti1 votesIt's stupid that you have to choose in the first place. Definitely a disappointing event.
TitanAnteus1 votesIt's stupid that you have to choose in the first place. Definitely a disappointing event.
Dawnbringer Riven vs Nightbringer Yasuo was the same? What are you guys on about?
Quepha1 votesThose were based on the icon you equipped, this is based on the game modes you're playing in. That means you may have to avoid playing the game mode you like if you prefer one path's icons over the other.
Mysticman891 votesI suppose it's a little bit of a pain, but at least the first patch mission for each thing is pretty quick to complete at least, and then once you're on the path you want you're free to do whatever.
And for those who don't have strong feelings about which path to go on, whichever path they 'accidentally' complete first is a reasonable way to choose.