I would avoid round mirrors, because the fireplace is rectangular itself and there's too much contrast.
But I like the first mirror, it's kind of rustic and modern at the same time. :)
However, if you have some specific ideas for a mirror in mind, you can even opt for a custom made one. Last year, when I was helping my sister with choosing the mirrors, we found these custom mirrors in Brisbane and they look amazing in her new living room. It's worth a shot, finding such stores in your area. You won't regret it. :)