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I always rent with Sixt instead of Europcar because in the past I had some issues with the Europcar. Sixt normally holds twice the rental price which should work perfectly fine with your card. With Sixt you may also have better chances to get a car upgrade which at least in my experience only rarely happens with Europcar. Do not count on an upgrade though. As pointed out by Alex there is no difference in renting from DUS or FRA. I strongly recomment not to decide the airport based on the rental car but on the airport and the airline product (airline, price, schedule, final destination etc.).