I rent with them a lot in Miami, and a Mercedes ML is a Mercedes ML. You don't get a "similar". I think all their luxury cars are MB or BMW. The one issue I had with them was being charged for a cut on a tire (had a MB SLK), which I am sure I did not make, but then again... who checks the tires before leaving with the car?
And also make sure you return a full tank of gas, cause it's really expensive if you don't (in Vegas you have a gas station on the car rental itself).