South Korean reasoning for choice; and as always best to read entire article at source:
S. Korea picks Lockheed Martin's F-35 as main fighter jet 22 Nov 2013 Yoo Seungki
"SEOUL, Nov. 22 (Xinhua) -- South Korea picked Lockheed Martin's F-35A stealth fighter as its main combat aircraft to better respond to possible nuclear and missile attacks from the Democratic People's Republic of Korea (DPRK), the Joint Chiefs of Staff (JCS) said Friday....
...Seoul-driven counter-attack strategy, which pre-emptively detects and intercepts missiles and nuclear threats from Pyongyang. Seoul and Washington agreed in October to first strike the DPRK if signs of Pyongyang's nuclear attacks are detected...."
S. Korea picks Lockheed Martin's F-35 as main fighter jet - Xinhua | English.news.cn