Yeh, there's not a lot of difference at the the very basic formula of porter vs. stout. A stout just gets a little bit darker and a touch more dry/earthy/bitter (although I don't like that term) from the darker, roasted grains used. A porter when held up to light should let some light pass through, but not a lot. A stout should be fully opaque.
From there, stouts just took on a life of their own with craft brewers I think mostly because of the name..stout just sounds much more manly than porter. Imperial stout. Chocolate stout. Oatmeal stout. Ect. Stout just had a better marketability.
Much like IPA's and the hop wars that continue with them. It just got a niche and brewers ran with them for name appeal.
As for answering the question...in the summer give me a traditional wheat beer and in the winter a stout of any variety and I'm happy as a clam.