I was interested in the OP's question recently, too, as I'm planning a RTW that involves three stops in ICN. I found a thread here in the DL forum and some info on the KE forum, as well as opinions from a few other web sites. The general consensus (nearly unanimous) was that KE is superior. So I opted for KE for that reason plus the fact that it may be the only chance I'll ever have to ride upstairs on a 747.

As it happened, I ended up with the 747 flight from LAX instead of ATL, though. I believe I had read somewhere on the KE forum that all J seats on KE flights to the US are the new lie-flat seats, although some of the old-style J seats still fly on other routes. I hope that info is reliable and I won't get the old seat on my LAX-ICN flight. I can confirm from the experience of a coworker that the ATL-ICN flight has the lie-flat seats.