Handle said:
http://geekbeer.com/articles/the-difference-between-porter-and-stout/
Thanks for the shout out.
Stout used to be simply the name for strong porter (or strong beer of any sort): today,with many beers described as "stouts" being weaker than some beers described as porters, that distinction has been lost, and there is no meaningful line - certainly not a historically meaningful one - that can be drawn between modern stouts and modern porters.
While this might be true for specific examples, I would contend that there is definitely a modern distinction between stout and porter that is useful to the modern beer buyer. Historical examples seem worthless in this context.
In reality, most items on the shelf labeled "stout" are going to be heavier than most items labeled "porter" (both in mouthfeel and ABV), and most stouts are going to have a roasted character that most porters will lack.
Additionally, while you might be able to take a Porter from Brewery A and a Stout from Brewery B and reasonably contend that they are both actually porters, both stouts, that the stout is weaker than the porter, or that they violate the guidelines in other ways, it's a reasonable assumption to make that
from a given brewery, their stouts will be weaker than their porters.
(One interesting exception here is Avery, whose standard Stout is weaker than their Porter, probably because they have giant stouts in their Demon line and don't feel the need to make a big stout available all the time.)
Anyway, saying there is no difference to someone that is asking because they want to know how to buy beer is just disingenuous. There are lots of differences that are useful in the modern age.