Does field type matter in a MongoDB index?

Here are two examples of a document structure in MongoDB.

{  
 UserId: "123",  
 UserName: "Usain Bolt"  
}



{  
 UserId: NumberLong(123),  
 UserName: "Usain Bolt"  
}

If I were to create an index on UserID, would there be a difference in find query performance between the above two examples?