Thanks to everyone for kind and helpful comments.

I thought about that - to stay near airport and go into town for sightseeing. However, then I'll have no base in town, to relax or drop any shopping off etc.

I was attracted to GMP-HND because of convenient locations of both airport (as well seeing these airports), but the noon flight has not had any seat open for several days. Obviously, many others prefer it too. So my choice is between 8AM GMP-HND or 1:25PM ICN-NRT.

I'd be in Seoul for only one day after a very long journey ORD-NRT-HKG-ICN --- thankfully I''l have lounge access and ORD-NRT and HKG-ICN are in Business, but still reaching ICN at 5AM and trying to experience the city all day, an 8AM flight is probably not the wisest choice. If availability doesn't change, I am leaning towards the 1:25PM from ICN.

I assume there is good train connections to either airport. Of course ICN is farther, but being able to wake up normally in the morning seems more attractive tight now.