There are multiple differences between Defoe's A Journal of a Plague Year and Pepys' Diary. Most of these differences come down to one principle factor: one is a work of fiction while the other is a factual account of events. Defoe's piece was written many years after the plague and is focused on a fictitious character's perception of the events that took place during the year 1665, while Pepys' is a true account of his own life during the plague of 1665.
The most noticeable differences between these works is the level of attention drawn to the plague and the amount of personal information shared in the texts. As mentioned in my last blog post, Defoe keeps H.F. focused on the events of the plague and provides little in terms of personal context. Pepys, however, is mostly focused on his day-to-day life and generally only mentions the plague in passing. This is indicative of Pepys reality, as he would have been doing other things throughout the year than simply hyper focusing on the logistics of the plague.
Further differences appear through the syntax. Defoe has fully structured sentences. Pepys' work is completely different, as it is primarily composed of sentence fragments. The reason for this is tied to the fact that Pepys' writing was a true diary that was never meant to be read by the general public. Due to the intentions of publication of each work, they have very different levels of readability through their grammar and syntax.