Could you give us more of a sense by what you mean by "overrated" and "influential?" Do you mean simply in terms of their personal writings? Do you mean in terms of intellectual projects (both men were actively involved in compiling the Encyclopédie)? Do you mean in terms of events from their lives or their personal connections?

In a lot of ways, Diderot and Voltaire are very different figures that were concerned with different topics. Voltaire was known, then and now, as a bit of a crusader against fanaticism, while Diderot was much more interested in materialism and sensory perception (Diderot's le rêve d'Alembert is largely an attempt to theorize how the human senses work). In some cases, judging which one was more influential (again, you'd have to specify regarding what or for whom) is like comparing apples and oranges.