If KE showers are full - do check out the Rest & Relax area free showers (show both BP to get free entry as transit, only 3,000W if originating passenger)
Matina, SPC, Lounge L, Matina Gold - Korea has caught on to the credit card lounge access 'game." Tons of people have PP here and these lounges get full. People say Matina has better food, though. But IMO as an American who loves Korean food, eats it daily and is "used to Asian style American food," I can't tell a difference. KE Lounge food isn't bad. It's just not Cathay Pacific Hong Kong lounge quality which is why people knock on it at ICN.
Ultimately, it depends on what time you're flying out.