Since your purpose is building up your portfolio, I think you should go with Unity and get working as soon as possible. I heard people in the industry like using Unity, because hiring people who already have experience with it is easier because they don't need to train you as much and everything. That's just something I heard on the Internet though.
About digging deeper into C++, it depends on you. Developing your own framework takes a lot of time and sweat. If you're impatient and want to get results quickly, it might make you really frustrated, but if you're more interested in the study of programming, then it might be worth it.