I have not personally flown either, but the general consensus is that the KE 748 F seat is perhaps the best seat out there. I don't think the Asiana seat is bad per se as their A388 is a newer one within the past 2-3 years.

Also, from my experience, KE has a more consistent on board product. Even if they swap planes, you shouldn't be too surprised by the new one. Whereas say, Asiana, if they swap suddenly (which they might of A388 is out of service) you could go to an old 747 with a much more inferior seat.

I do think Asiana has better service and catering...again, from a Y perspective. I know that others say the whole point of F is better service.

In the end, I would probably go KE because of the seat. Caterimg differences in F would be minor. And it's not like either are going to give you bad service or be rude, especially compared to America.