What’s the difference between initializing the object member variable m_A in-class versus (class B) in an initializer list (class C)? Is one method more efficient or generally preferred over the other?

class A
{
public:
    A(int num) { m_Int = num; } 
private:
    int m_Int;
};

class B
{
private:
    A m_A{10};
};

class C
{
public:
    C(): m_A(10) {};
private:
    A m_A;
};