Better to return a C++ reference or a weak_ptr?

class Member;

class ClassWithWeakPtr
{
private:
   boost::shared_ptr<Member> _member;
public:
   boost::weak_ptr<Member> GetMember();
};

class Member;

class ClassWithCppReference
{
private:
    Member _member;
public:
    Member& GetMember() {return _member;}
};

const Member& GetMember() const;
Member& GetMember(); // ideally, only if clients can modify it without
                     // breaking any invariants of the Class

weak_ptr< Member > GetMemberWptr(); // only if there is a specific need
                                    // for holding a weak pointer.