The Singapore flight arrives very late at ICN, so you have to rely on late night buses at infrequent intervals OR pay an expensive taxi around 70-100 USD depending on where you're going and what time you actually get in the taxi. All Korean taxis start higher after a certain time (I believe 12, I never bothered to check) and are more expensive "late night." This difference is negligible in town but going somewhere super far like ICN can mean it will really add up fast.
Korean Air and Asiana will both run A380s and Asiana's are newer. Korean Air's 380s have more per row, but often the cabin is so large (entire 2nd deck) that it is never full and people have a lot of space.
Asiana does tend to sub aircraft suddenly and the onboard product can change to a much older product. Korean is consistent in that all of their long haul aircraft feature flat beds and consistent on board product.
Others say Asiana's soft product, service and food, are much better. Asiana's 380s are brand new (1-2 years) compared to KE's which have been around a few more years.
Having only flown Y, I cannot vouch for business. I have been pleased with both KE and OZ, and I would say that the general assumption above is true.
There are two JW Marriott. The one in Gangnam is attached to the Bus Terminal complex and is convenient for subway travel, BUT the subway station is very large, confusing and busy. Might be more hassle. The newer one in Dongdaemun is much smaller and in a much more traditional, older Seoul neighborhood. It is closer to markets, tourist attractions etc. The Gangnam one is near Gangnam nightlife, but you will still have to get to/from Gangnam every day. For business, though, Gangnam is better.