Generally speaking, Sixt has been quite good to me so far. They are my preferred rental car company so far and I have never felt the need to go with someone else.

Cape Town Airport, PDAR (Merc C180) and received that one as well...
Due to the lack of upgrade possibility (as it was the largest they had) the lady at the counter offered to only bill the lower class. Gratefully accepted but unfortunately the final bill did not mirror this offer. Shame.
One strange thing was that the agent told me their cars do not have sat nav and yet I enjoyed the Mercedes Command System.
The car itself was excellent. Slightly over 10k kilometers and surprisingly many dents etc. but that's only aesthetics.

Johannesburg Airport, SDAR (Ford Focus Automatic) and received FDAR (BMW 320i). Decent car, only 16k km on the clock, no Nav unfortunately. Main drawback, the engine is just too weak for the size of the car.

Just today in Amsterdam RAI, rented MCMR (Renault Twingo, Fiat Panda) and received CDMR (BMW Mini Countryman).
Didn't drive the car myself as it's for a friend who doesn't have a credit card but from what I saw, it's a totally acceptable upgrade.