The train is great if you don't have too much luggage. If you do then there's a bunch of escalators, elevators, and a txfr you need to deal with.
Taxi is faster but you might have a hard time fitting all your luggage into the regular taxis ($50) in which case you have to take the luxury black cabs that cost around $100 into the city. When I travel with my family of 3 we sometimes have to take the jumbo black van cabs that cost even more.
Most people I know who go to Seoul frequently stick with the KAL Limo for a reason especially out of T2. So much more convenient and comfortable ride. Also very frequent with stops to most big name hotels.
https://www.kallimousine.com/eng/schedule_en.asp