hi, im a platinum member at sixt, and i want to go on vacation in Europe, but the most important part of the vacation is the car, so tell me you're experience, which stations or were did you get the best upgrades?, and like this ill decide were i would travel, Germany, France, Italy, Switzerland etc.
German offices should also give reasonable upgrades but not always. I suggest you book the car you will be happy to drive and then anything better is a bonus. Relying on an upgrade is a path to dissapointment
I have made very good experiences many times at BCN, except for the current rental, when we got exactly the Golf that we booked (and at exactly the same rate that anybody else would get, no trace of any Plat discount). Also made good experiences in VIE.