I've been in both and prefer the one in Concourse J. Roomier and better atmosphere. The beverage setup in the lounge in Concourse F was self-service while the one in Concourse J had someone behind a bar to give you your beverage. The one in Concourse J had sandwiches cut in 1/4s (F had no food other than cookies and maybe some fresh fruit), as well as a better selection of snacks than the one in F. The one in F is a long walk down the Concourse. The one in J is closer to the beginning of the Concourse. Depending on what concourse your flight is, you may have to go through security once to get to the lounge, then again when you go back to your gate.