What is the difference (in content) between Nefesh HaChaim and Derech Hashem? Are they written to educate the reader on the same topic? Does one presuppose knowledge while the other doesn't? Is one a stepping stone to the other?
Slightly edited from Wikipedia articles:
Derech HaShem (The "Way of God") is a philosophical text written in the 1730s by Rabbi Moshe Chaim Luzzatto. It systematizes the basic principles of Jewish belief regarding the existence of God, God's purpose in creation, and the logical consequence of other concepts in Judaism. The reader is led from thought to idea, from idea to a logical whole of the structure of Jewish belief.
In my experience, it is better learned from a Maggid Shiur than alone.
Chaim of Volozhin was attracted by the fame of the Vilna Gaon, and he became one of his most prominent disciples. Reb Chaim Volozhiner 's major work is the Nefesh Ha-Chaim ("Living Soul"). It deals solely with complex understandings of the nature of God, and also with secrets of prayer and the importance of Torah, the purpose being "to implant the fear of God, Torah, and pure worship into the hearts of the upright who are seeking the ways of God". It presents a clear and orderly kabbalistic world-view that addresses many of the same issues as the Hasidic texts of the day.
It is written from a clear non-Hasidic viewpoint. There is a great deal of kabbalah. If you want to get some understanding of the kabbalah you need either a chavrusa with experience in that area or to hear a shiur. Another option is to study this English translation that skips the kabbalistic bits.
I agree with @kouty that Nefesh hachayim is more polemic, with lengthy explanations. Derech Hashem is concise.
Derech HaShem is easier to study and should be learned before the Nefesh Ha-Chaim.