Keep in mind that McDonald's split the DR into three territories (SD, the North, and the East); with each being exclusively franchised to different companies. So, while technically whether you eat at a McDonald's in Santiago or in Santo Domingo or in San Pedro, its all supposed to be the same; you are in fact buying from three different franchise companies and that can often manifest itself in different quality / promotions, etc.
Having said that, the whole country is one giant Burger King territory and as many of you probably noticed, Burger King seriously outnumbers McDonald's in the DR.