Hartman is a really accomplished throwing quarterback. From what I can tell, he's really good. He's thrown interceptions but he throws downfield a lot. He completed more downfield throws than Pyne even attempted this year and that is going to lead to some.
I like Buchner, but I don't think he's developed enough as a pocket QB in the two and a half games he's played to beat Hartman in a QB competition. But he'll have his shot.