My very first rental with SIXT (no status):
Aachen
FDMR (MB C200)
LDAR (BMW 530d)
again, no status:
Aachen
LWAR (BMW 530d Touring)
XFAR (BMW X5, horrible car)
bahn.comfort
Aachen
LDAR (BMW 530d)
PDMR (BMW 520d), opted for downgrade as I needed a BMW (they offered me a Lexus, but no time to read the manual) and they only charged the lower PDMR fee
Seems to me that you are likely to be upgraded (or receive a totally different car) when renting from smaller stations. The "best" I've ever received was a fully-featured MB E350 CGI with a Brunswick (!!) number plate, although I didn't really like the that car... Pity they don't have the Audi A6 3.0tdi any more.