Flown DeltaOne from SEA pre-covid to Japan many times, but not after covid. Flown both A350 and A339.
Recently flown KE 77W from SFO-ICN in Prestige.

I am probably in the minority here when I say I prefer the DL1 seats and cabin temp. LOL. The DL1 seats are more private and the seat itself is a bit softer. Cabin is cooler and more conducive for sleeping. But I won't pay $800 more to fly it over KE.

KE seat is a tad bit wider and has much more legroom. Seat and padding itself is too hard for me. Meal service is way better than any that DL is capable of serving but by no means is it worth going out of the way for. Flight attendants are all very pretty, petite and well-kempt. But service is a bit robotic. Entertainment offerings are quite limited - it is almost as if it was the early 2000s where having just a few movies alone is a novelty. I am not impressed with ICN airport. Prestige Lounge is very very crowded.

Lastly, aircraft swaps can happen. I booked my flight 8 months out, and I routed it through SFO to experience the 748. A month later, aircraft got swapped to a 77W. My connecting was on A330/ 77W but those got swapped as well for the dinosaur of a 777-300A!!! It was both a treat and a curse flying on an antiquated version of the 777. I heard that SEA can get aircraft swaps to the old style 2-3-2 lie-flats instead of the 2-2-2 suites sporadically. KE also likes to fly their planes even with some inoperable toilets.