Doesn't seem relevant to OP, but Fly America doesn't apply to foreign-foreign flights. There's no "short flight" exemption domestically, though, but there are other exemptions like convenience of travel time.

Given the number of variables with regard to flight delays, customs, baggage, etc. I don't think I want to guess which train I can make and book before the flight. Once I'm on the ground I'll see what I can get. Maybe once I grab my bags and clear immigration. The good news is that I have all day on Tuesday to reach Busan, so lots of flexibility.

My specific problem was that there were all Delta flights available, but they generally required that I leave early Sunday morning and then spend an extra night in Busan on arrival. The travel office thought they were fine, but since I'm supposed to return from another trip the Friday night before I didn't think so. This was the best alternative I could find that let me leave Sunday evening instead.

Flight or Train PUS-SEL
 

What's the conventional wisdom these days whether to fly or take the train from the Seoul area (Myeongdong, not the airport) to Busan (Seomyon area)? I'm not concerned about price per se, especially as the price for both flights and train is quite nominal. I just prefer the most convenient, enjoyable, and timely option. Air Busan flights are quite cheap at $30 USD or so each way (Korean Air flights are about $90) and the KTX train is about $48 USD for economy or $65 USD for first class.

When factoring in distance to the airport, time in security, and typical delays (i.e. I assume flights are more delayed than trains), would both flights and train be roughly equally good? If so, I'd probably take the train 1 way for the view and experience. Does first class offer anything incrementally better or just a larger seat like in Europe?

Having tried both recently, I would take the train every time. Very frequent departures, comfortable seat, less travel time to/from train stations compared to airport in both ends.

Don't expect too much from the view, a lot of the trip is in tunnels.

Thanks! What's the difference between economy and First?

Couldn't say - I was perfectly comfortable in standard class.

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That's great! Thank you!

KTX First class; you get a wider seat. It is arranged in a 1-2 seating configuration, as opposed to a 2-2 in economy. 'A' seats are the solo seats. It seems a bit quieter in First Class. It might be the carpeting that absorbs some sound, whereas Economy has a rubber flooring. Also, First Class passengers can get a free small bottle of water from the vending machine nearby.

I recommend taking First Class. I find Economy a bit too cramped width-wise. I'm not even considered a big person! But the extra space and quietness in First is appreciated.

Perfect! Just wanted to confirm that it was worth the incremental few dollars extra.