I'm a bit confused. EK is the only airline that offers first class from Arlanda. If you're flying EK first class, you have access to the Stockholm Arlanda Lounge after passport control in the F-pier. Your Amex gives you access to Amex Lounge by Pontus, which is really only a secluded part of the Pontus in the Air restaurant. The Amex/Pontus probably has the best food of all lounges at Arlanda, but it's also the least spacious and least private.