Both mean the dog’s head, but:
• die hond se kop is the everyday, idiomatic way—short and natural.
• die kop van die hond literally means “the head of the dog” and feels more formal or emphatic.
Choose the first for casual speech, the second if you want emphasis or a more book-like style.
die = “the” (definite), ’n = “a/an” (indefinite). Use die when you talk about a specific dog known to listener; ’n when it’s any dog:
• Ek sien die hond. = I see the dog.
• Ek sien ’n hond. = I see a dog.