I've rented from Europcar and picked up at FRA. If you can pick up from an off airport site, it'll save you 50 Euros or so. Also most rentals in Europe are manual but you can get automatic for an extra fee. I reserved a A3 wagon with navigation but when I got to the airport they "upgraded" me and gave me a choice of a VW Jetta or another European brand car I can't remember maybe Puegot. Took the Jetta because the attendant told me that the navi may not be in English on the other car. It was a decent car and held up well on the Autobahn but I would have rather had an A3. Overall I'd say Europcar is fine and any status with National won't matter because they have their own loyalty program.