The first impression - The Buildings - is political and governmental exposure, written during Justinian's lifetime. This kind of text was produced to promote the emperor's deeds, as a way to show to the further readings who was the Emperor. It's a public story wrote by a historian.
However, the second impression is personal. Procopius was seeing the emperor's private life, his behavior, his way to deal with the problems, and his temper. Procopius analyze Justinian's moral and conclude that he was a good person, but with some problems to control the anger and the ego.
I think that the main difference between these two excerpts is not only their nature - private and public - but also when they were written. The first is a public promotion during the emperor's lifetime, as a way to show his importance and glory. And the second was published after Justinian's death, and Procopius was not worried about reprisal. It's a personal point of view about private life.
Explanation:
Look at these excerpts, and see how Procopius describe the emperor:
#01: "In our own age there has been born the Emperor Justinian, who, taking over the State when it was harassed by disorder, has not only made it greater in extent but also much more illustrious, by expelling from it those barbarians who had from old pressed hard upon it. . . .Justinian created countless cities which did not exist before. And finding that the belief in God was, before his time, straying into errors and being forced to go in many directions, he completely destroyed all the paths leading to such errors, and brought it about that it stood on the firm foundation of a single faith."
On the other hand in the second text we have:
#02: "Justinian was dissembling, crafty, hypocritical, secretive by temperament, two-faced; a clever fellow with a marvelous ability to conceal his real opinion, and able to shed tears, not from any joy or sorrow, but employing them artfully when required in accordance with the immediate need, lying all the time; not carelessly, however, but confirming his undertakings both with his signature and with the most fearsome oaths, even when dealing with his own subjects."
