Since the topic is timely, let me redirect readers to Dr. Jared Farley's classic post he did at American Creation on the matter(s).
Jefferson, in part, created his "Syllabus" as an outline to assist Dr. Priestley's writing of a book Jefferson suggested he undertake.
A second purpose of Jefferson's "Syllabus" and Priestley's book was to help the cause of primitive Christianity in the United States (Remember what is happening in New England at this point between the theologically liberal Unitarians and more orthodox Congregationalists.)
A third purpose of Jefferson's "Syllabus" was to provide some family and friends in his inner circle a better understanding of his version of Christianity.
In similar fashion, Jefferson created his "Philosophy" as a draft to be utilized by Dr. Priestley in his upcoming book comparing the moral philosophy of Jesus with those of the ancient philosophers and Jews. It is unknown if Jefferson found out about Rev. Priestley's death prior to composing his "Philosophy" and if that had any impact concerning the composition of that work, which was finished before and professionally bounded on March 10, 1804.
Jefferson was planning on utilizing Dr. Priestley's "Harmonies" in composing his "Philosophy", so it may not have entirely been Jefferson's original work or a direct statement of his theology. (Again, we don't know if Jefferson's plans changed, or if the text of "Philosophy" changed once Jefferson found out about Priestley's death.)
If Jefferson stuck to his original plans concerning "Philosophy", then that work is a mixture of Jefferson and Priestley's views and not simply Jefferson's. Plus, it was intended to serve as an introduction for Priestley's upcoming book, so it had to be acceptable to the general American reading audience.
If Jefferson stuck to his original plans concerning "Philosophy", then his "Life and Morals" might be a better indicator of his theological views since: 1) He had more time to work on it; 2) He was not writing in conjunction with someone else; 3) He never intended "Life and Morals" to be published under his name or anyone else's, and therefore, could be more honest.