I've now flown both Delta's A350 Delta One Suite (DTW-NRT) and Korean Air's new Prestige Class configuration on their 777-3ER / 77W (SEA-ICN) and thought I'd provide a comparative review in case anyone is trying to decide between these options.

PDB
KE offered orange juice, guava juice, and water. I imagine alcoholic PDBs would have been available upon request, but I was surprised they didn't offer bubbly. Delta offered the standard PDB options: orange juice, water, bubbly.

Meals
This is kind of a wash. KE had two full meal services (lunch after departure and dinner before arrival). Delta had one full meal service (dinner after departure), a small plated snack mid-flight, and then pre-arrival breakfast (which didn't really make sense to me since we were arriving in the evening local time). Delta's meal service was perhaps a bit more thoughtful (warm nuts, multiple appetizers instead of just a salad as an appetizer KE). DL had more wine options and better desert options. I liked KE's entrees better but I imagine this is part luck.

Seat / hard product
KE clearly wins here. Don't be fooled by KE's 2-2-2 configuration; because of the unique design of the seats, every seat has direct aisle access. I was in a window seat, which was very private and felt like a suite. Unlike many business class seats, KE's new 77W seats don't have footwells that go into the seat in front (as with DL's A350 suites). Rather; the seats have a rather large ottoman. This means that the seat is basically the same width the full length of the bed. The seat also had a large storage compartment above the ottoman, as well as additional storage by the armrest. Even though there's no "door," there's a motorized privacy divider between adjacent seats. I can see how someone who likes privacy and doesn't need much space might prefer the A350 suite versus an aisle or center seat on KE's 77W, but if you can snag a window seat on the KE 77W, you get the best of both worlds. I found Delta's A350 suites to be sufficiently private and didn't mind the cozy feel (some people would call it claustrophobic), but it did feel cramped, especially due to the narrow footwells. Delta's cabin and seats are definitely more stylish, but that's not that important to me.

Bedding
Delta wins here. I was honestly surprised by how small the KE pillow was, and the blanket was not very soft and awkwardly heavy. This was a sore point for me.

IFE
Both Delta and Korean Air's IFE screens are immovable, and so you can utilize them from gate to gate. With the touchscreen remote, KE's interface was better. I was surprised, though, by how few movie options Korean Air had compared to Delta; plus, even KE's western movie selection were a bit esoteric.

Service
The service was really good on KE. And despite what I've read elsewhere, all of the cabin crew's English was great. Service on Delta can be great, but it can also be irreverent, which I think is generally true of American carriers versus carriers in other areas of the world, especially Asia.

Overall, I liked Delta's soft product better but thought that Korean's hard product with the new 77W configuration is vastly superior to Delta's A350 suites.