i have the following two class which convert object into xml string

should i do something like

class Person
{

   public string GetXml()
   {
       //return a xml string
   }  
}

or it is better to create another class which accept the person as a parameter and convert it into XML something like

class PersonSerializer 
{         
     public string Serialize(Person person)
     {
           // return a xml string
     }

 }

Thanks