Korean Air travel time will be about 10.5 hours, including 2 hour layover in Colombo. The economy cabins are clean, bright, well kept. Service is polite and hospitable. I think you will enjoy.

Personally, I've never flown Qatar, but I'd imagine the service to be similar. If the price for Korean Air is fair, I'd avoid a layover in Doha. Unless, of course, you earn miles on Qatar.

Also, on Qatar, your flight itself is much longer. 13 hours or so with a three hour layover. In total, six hours longer. Both are overnight flights.