I have just flown in F for the first time from LHR to Seoul and am returning in J, again a first time experience, indeed I am sat in the Asiana lounge now at Incheon.
I found the whole F experience to be excellent, cabin crew on both sectors were great, the food and wine were both really good. From LHR to DXB I had the full lunch service, with Dom Perignon and good wines, on DXB to Incheon I slept and had a continental breakfast prior to arrival. I found the suites to be great, obviously private and very good for sleeping. I didn't use the shower but enjoyed the spacious size of the "bathroom". I spent sometime in the J bar and was the only person in there, the purser brought me Dom from the first galley. I had a three hour layover in DXB and thought the lounge great, shower and a light snack.
So I am looking forward to see how J compares on the way back but am sure it will be fine too.
Whichever you choose I am sure you will enjoy.