I just went out for a run with my dog, and I'd like to change my mind. I think in the U.S. we usually call of Mercedes Benz a Mercedes. I lived in Japan for many years, and they call a Mercedes a "Benz".
On this website, feel free to call them whatever you want as everyone will know what you're talking about.
If you talk to any American in the general public and call your car a Merc, they will automatically assume you're talking about a Mercury.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".