Daytime for a full service experience. For instance, at night departure, they serve a meal right after take off, but you can't really enjoy it as much because everyone wants to sleep.
You can see the service schedule here:
https://www.koreanair.com/korea/ko/t...-americas.html
You'll see that both flights serve a meal after departure and then also around hour 6-7.
Also, personally, I find that the jet lag from the daytime flight is much better. You can leave on normal American time, enjoy your afternoon on the plane. I always take a sleeping pill after the first meal and sleep around 5-6 hours. Then you land in Seoul in the evening and it's just a short while to stay awake before bedtime. Typically, the jetlag is gone right away.