7,5 hrs - definitely head downtown.
No matter how good the PlatWing is (and it's good and better than the J lounge), it's still an airport lounge. The train from under the airport is something takes 25 minutes, costs 4,10€ oneway, runs every 10 minutes. (you might take either direction, the P train (clockwise) is 2 minutes faster than the I train (counter clockwise)