It's to be the F-35:
South Korea's Joint Chiefs of Staff were expected to endorse an "all F-35 buy" of 40 Lockheed Martin Corp F-35 fighter jets and an option for 20 more at a meeting on Nov. 22, two sources familiar with the competition said on Friday. ...
One source said South Korea was sticking to its initial plan to buy 60 jets to preserve the terms of an industrial offset package that accompanied the Lockheed offer and included a satellite to be launched and placed in orbit.
I figured that the South Koreans could use numbers more than stealth to defeat the North Koreans and that the frontal stealth of the F-35SE was good enough to face China's frontal-stealth only planes they are developing.
It may be that South Korea feels the F-35 deal gives them access to technology they'll need to stand up to China in the future. They certainly don't want to be in Taiwan's position of having to buy equipment from countries under pressure from China not to sell to Taiwan.
You never know when China might discover that the Korean peninsula is a "core interest" of China, eh?
