The KE lounge at NRT is horrid, but at ICN the one in the main terminal is very good. I'm not at all a fan of the KE lounge in the satellite terminal at ICN, partly because it seems to be all white and turquoise plastic with no comfortable seats. However, the satellite terminal seems to be used for partner flights, so I would guess that you would arrive and depart at the main terminal on KE operated flights.

At NRT, as SkyTeam ElitePlus you can use the DL lounges instead of KE. The gates aren't very far, although in IROPs, you might want to relocate to the "native" KE lounge to keep up to date on changes.

Personally, I'd pick the DL flight for the direct aisle access and, in particular, to avoid the risk of the middle seat in the 2-3-2 configuration. The A380 with 2-2-2 is a bit better, but unless it's guaranteed on both segments, I wouldn't go all the way out to NRT for it.

I'm responding without knowing the times of the flights that are involved. IMO a late night HMD departure is good, but early morning not at all.

However, you should be award that the DL HND flights don't get the full BE F&B treatment. You will have only a few choices for supper, which is served all at once on a single tray. Wines on this route are clearly inferior too IME, compared to BE wines on other long haul routes. What you'll get is similar to the NRT-SIN flights in BE.