Seoul and Busan in one trip: how many days would you give each?

“Is Busan worth adding?” is one of the most common planning questions for a first Korea trip, followed straight away by how to split the days.

What people usually weigh

Travel time: the KTX makes it a comfortable train ride, but it still eats half a day once you count checkout, getting to the station and checking in again.
Packing and moving hotels: some travellers find one move fine, others say it breaks the flow of a short trip.
What you want: Seoul for palaces, shopping and neighbourhoods; Busan for the sea, beaches, seafood markets and a slower pace.

Patterns that come up a lot

Short trips often stay Seoul-only, or add Busan only if there are enough days.
Longer trips split both cities, sometimes flying home from Busan's airport to avoid backtracking. Check your airline options before booking.
Book KTX seats early for weekends and holidays.

If you have done both: how many days did you give Seoul and Busan, and would you change the split?