You may want to consider where in Busan is your final destination. The Busan/Gimhae airport is located outside of Busan. If you are staying near BEXCO for a conference, the Busan rail/subway station is closer, but still a good 30-50 subway/bus/taxi ride to BEXCO area. If you fly in to Busan/Gimhae, well there is most likely an airport limo bus that can take you to BEXCO area. It will be an hour or more.

If I was in your situation, I would take the plane. Several reasons. No need to move luggage onto a train and make a transfer with luggage at Seoul station. Also, immigration can be done at Busan/Gimhae airport (assuming you have checked your bags all the way to Busan). Immigration at Seoul can get busy at times. Busan's international airport rarely gets crazy. Lastly, the airport limo bus from Busan/Gimhae airport to your hotel is usually stress free and relaxing. The chairs in the bus are comparable to domestic(US) business class seats.

Having said this, if you want to take a scenic tour though Korea, I recommend a window seat on the KTX (first class). Cherry Blossom trees are now blooming.