Let’s dispense with the nonsense right away: anyone who thinks the answer here is “stouts have roasted barley” can leave now.
There’s a rich blend of history and anecdote surrounding the development of porter and stout from the eighteenth through the mid-twentieth centuries, particularly as it pertains to the differences between the dark beer products of London vs. those of Dublin on the far side of the Irish Sea. Attempts to draw the dividing line based on the grist used are destined to fail since historical porters and stouts each used pale malt, brown malt, chocolate malt, patent malt, and/or roasted barley, depending on the year and the brewery.
However, there is one definitive fact we can extract from the available historical information: in the years leading up to the twentieth century, when a brewery produced both a porter and a stout, the stout was the stronger of the two beers. There also existed, for a time, a thing known as “pale stout” (not to be confused with modern interpretations of pale/white stout), and it was likewise a stronger version of pale ale. Stout, then, was an internal (brewery-specific) differentiator of relative strength and little else. “Stouts” were generally stronger, on average, than “simply porters,” even between breweries, but it wasn’t guaranteed.