Rental reservation for one car and it was changed for another -- SIXT

I saw a nice deal on dealnews dot com about Mercedes C class rentals in Florida. Went link to the site and was delighted to see a full list of cars to choose from. Jaw dropped to the floor when I saw that a Mercedes E class was being offered for $360 for the week all taxes and so forth. Hertz rents the same car for $800/wk.

I noticed the other day when checking in on my reservation that they downgraded the reservation to a Mercedes C class. I feel like that was wrong for them to do because they gave me a choice and I agreed to rent a E class and the advertised price. I got both email confirmations for the Mercedes E class. What would the educated consumer do in this situation. In a way I don't want to call them and argue now because I don't have any other reservations, so don't want them to drop me all together.

Any input on how to work this will be very appreciated.