I rent a lot cars at Sixt (sold my own car a year ago) and always a city station on a short time frame.
If you choose options like navigation and automatic transmission, the chances to get another car than requested are quite good – and I am only Gold.
From my experinces with them in the past, it was quite good to get another car than booked if you do this only a short time in advance. Since a few month, it seems, they have upgrade the system showing the availability of cars, because some groups are not available at every Sixt Station and some Stations show no availability for requested dates.
My experiences with them in this year (reservation was made the time 10 - 25 hours in advance); but in this short time frame not every car is available.
I book always the lowest or second lowest available categorie or a special group if I for example need a station wagon. I always choose as option Navigation and Automatic transmission (I know, a Fiest is not availble with nor Automatic neither Navigation)
Sixt Düsseldorf Central Station
booked Ford Fiesta (ECMR) got VW Golf
booked Mercedes A (CLMR) got Volvo V 50
booked Audia A 2 (CDMR), got Mercedes CLS automatic transmission
booked Audi A 3 got Audi A 3
booked VW Passat Variant (SWMR), got BMW X 3 (but rejected, as an SUV is not covered by the insurance included in my credit card and got as I need a Station Wagon a Ford Mondeo (downgrade)
booked Audi A 3 (IDMR) got BMW 1 Series (no upgrade), which had after an hour driving a breakdown (electronic malfunction, no ESP, ABS, Power Steering, etc), changed to a SLK for the rest of a sunny weekend
Sixt Frankfurt
booked Ford Fiesta (ECMR), got Mercedes A
booked VW Golf (CLMR), got Mercedes C with automatic transmission
Sixt Moers
booked Audi A 2 (CDMR) got Mercedes A
Sixt Berlin
booked Smart Forfour (IVMR), got Audi A 3