What is the best way to travel between specific cities in Korea?

The best choice really depends on the route, so here is a practical rule of thumb. For major city-to-city trips, the KTX high-speed train usually wins: Seoul to Busan is about two and a half hours, and Seoul to Daegu, Daejeon, Gwangju, and Mokpo are all fast and frequent, running city-center to city-center so you skip airport hassle. Book on the Korail app (or the SRT app for trains from Suseo in southern Seoul) and grab early-bird or off-peak fares to save. For places the KTX does not reach directly or for a cheaper option, intercity and express buses are excellent: comfortable, covering far more towns than trains, roughly half the KTX price, and departing frequently from central terminals; book on the Kobus or Txbus apps or at the terminal.

Flying is generally only worth it for Jeju Island, since there is no train and the ferry is long; the Gimpo to Jeju route is one of the busiest in the world with cheap low-cost carriers. For most mainland routes, flying wastes time once you count airport transfers, so the KTX beats it. Driving a rental makes sense when your destination is rural, spread out, or poorly served by transit, like touring the countryside, coast, or national parks, giving flexibility, but watch for expressway tolls, holiday traffic that can double your time, and city parking. As a quick guide: big cities go KTX, smaller towns and budget trips go bus, Jeju go fly, and countryside or multi-stop road trips go rental car. For a first-timer, the KTX and express buses cover almost everything and are easy to book.