I think that both works are fairly brutal in terms of their view of the human predicament. In my mind, it is a subtle parsing between both works to see the presence of Realism and Naturalism. Essentially, the former drives the latter, which reflects how closely associated both works are to one another in their conditions and overall messages.
For Pere Goriot, there is an unmistakable statement of Realism that pervades the work. The condition of the elderly father who seeks only to win his daughters' love is one that takes Shakespearean tragedy and places it in the context of the Parisian real. There is not a glorification of the tragic condition, but rather a desire to depict it in the most realistic of terms and settings. This is not something restricted to the most elite of society. It exists, as Balzac says, in "our homes and hearts."
This is the same underlying sentiment in Zola's work. Yet, it is moved to another degree. Zola seems to be conducting a type of emotional experiment that analyzes to what extent individuals can do what they do. He arranges variables in such a way that it reveals how individuals are able to do what they do in the manner they do what they do. How can a woman who is emotionally detached end up participating in a murder- suicide pact? How can someone who is frail and weak end up exhibiting intense strength at the moment of calling? How can someone who is supposed to represent law and order fabricate a report for his own ends? In the final analysis, Therese Raquin is more of a Naturalistic work because it shows how individuals are able to represent and embody the conditions in which human beings can do what they are capable of doing.
I think that both works are similar in their bleak condition of being in the world. Both works depict a fundamental tragic condition almost intrinsic to human beings. Yet, it is in the idea of Balzac's Realism as one in which there is more of a general statement of tragedy within the human condition, and its detailing merely needs to be reported and assessed. For Zola, there is a Naturalist tendency to examine and place human beings in conditions that reveal a part of themselves that might not have been previously seen. It is a subtle difference in the sadness of both works, but a difference nevertheless.