If you have flown with both airlines and now had enough Chase points for
an award flight from ATL to BKK in Bus C which airline would you chose ?

KE: Outbound: 15.5 hours leaving at ~noon isn't my ideal kind of flying even if on an 747-8i but could also be on an A380 or 77W. It will maximize your J class experience but it's a long time during "awake" hours but I also an a night owl insomniac. J on the KE A330 isn't amazing but for a 6.5h flight you'll be fine in it.
Inbound:Flight time works better for the average person. A few hours nap on the BKK-ICN segment and then more rest on possibly a 747-8i on the return.

TK's flight departure from ATL is more ideally timed for rest in a nice seat, access to a nice lounge to refresh but then another longish flight of about 9 hours. The return has a late departure with the possibility of a nice layover in IST to tour the city.

I think it's more about what fits your travel style better and the equipment on the day of travel.

Thanks Yoshi for the thoughtful response.

The KE flight I checked where almost all on the 77 and the J seats do not
looked that great,But I failed to consider the long 9hr flight from Ist to Bkk
on TK. Since the TK cost 10,000 more points I am leaning towards KE,
except for the bad looking J seats.

Depending on which programs you're using for ticketing, assuming KE & UA, you could do 2 1-way awards also.

IIRC getting tickets with Chase points normally results in getting FF credit. Since the OP lives near ATL, using the trip to get redeemable miles and credit toward elite status with DL would be usaeful.