Include in header file vs. forward-declare and include in .cpp

I have a class B, and I want to call members from class A. So:

1.

//A.h    
class B; 
class A 
{ 
private:
    B* m_p; 
}; 
    
//a.cpp
#include "B.h"

2.

// A.h
#include "B.h"

class A 
{ 
private: 
    B * impl_; 
}; 

Which way is better and is this two similar when a small project with not too much dependence involves?