Should I initialize a member variable in declaration when it is initialized by a constructor parameter?

Which is recommended:

class Foo
{
    public X x = new X();
    public Foo(X _x)
    {
       x = _x;
    }
}

Or

class Foo
{
    public X x;
    public Foo(X _x)
    {
       x = _x;
    }
}

Please note in the second version, it is probable that I introduce other constructors where x may not be initialized!