I am visiting South Korea next month (last two weeks of April 2017) and, although I'm mostly staying in Seoul, I wanted to visit Busan for 2-3 days by taking a KTX train. I'm considering buying a Korail pass, which is cheaper than directly reserving/buying a roundtrip ticket. However, as noted by the link,
Seat reservations will be available after redeeming your Pass at the ticket booths. Pass- holders, who fail to make reservations, may use unreserved seats.
I'm planning to visit Busan during the weekdays, most likely a Tuesday -> Thursday trip. I arrive in Seoul the Friday before this, and this would be the earliest time I could make seat reservations.
Do I have to worry at all about the trains being too full? Would it be worth it to just forego the savings and reserve a roundtrip ticket to guarantee a seat?