The Seoul–Busan version works when Busan gets time for the places you want to see and the train day is treated as a transfer. If the week already has a concert, fixed dinner or late international flight, a Seoul base may give you more usable evenings.
Count complete days, not hotel nights
Arrival and departure dates are only partly available unless your flight times say otherwise. A city change also uses time for checkout, station travel, the train, luggage and another check-in. Keep the KTX date light and make the first Busan evening local.
When Busan earns its nights
Add Busan when its coast, port or neighborhoods are a priority for both of you. Two full days give room to choose an area and settle into its pace; a single hurried night can turn into two more packing and station transfers. If Busan is mainly a name on the route, spend those nights in Seoul instead.
A workable seven-day shape
One option is arrival in Seoul, two full Seoul days, a train and check-in day, two full Busan days, then a final night positioned for the departure airport. Change the order to fit your flights. Avoid putting a non-refundable dinner or performance close to the train arrival.
Keep the ticket and station aligned
Seoul–Busan train times, fares and seats depend on the date and service. Choose the departure station that fits your Seoul base, then check the live schedule with Korail before locking hotel nights. SRT services use Suseo; KTX services commonly use Seoul Station.
Compare the one-city and two-city versions against what you want to do together, not a target number of destinations. Leave at least one open block for a slow meal, a neighborhood you like or a change in weather.
Plan the train leg
Korail: live train schedules and tickets · Seoul, Busan or Jeju for a first Korea trip