I may be wrong but I've always considered porter to be be a sweeter, thicker brew whereas a stout tends to be thinner, drier and more bitter. Both can have chocolatey, coffee notes but IMO this tends to be more prominent in porters
The BJCP description of English porters and Irish stout have the following descriptions
Porter
Mouthfeel: Medium-light to medium body. Moderately-low
to moderately-high carbonation. Light to moderate creamy
texture.
Appearance: Light brown to dark brown in color, often with
ruby highlights when held up to light. Good clarity, although
may approach being opaque. Moderate off-white to light tan
head with good to fair retention.
irish Stout
Mouthfeel: Medium-light to medium-full body, with a
somewhat creamy character (particularly when served with a
nitro pour). Low to moderate carbonation. For the high hop
bitterness and significant proportion of dark grains present,
this beer is remarkably smooth. May have a light astringency
from the roasted grains, although harshness is undesirable.
Appearance: Jet black to very deep brown with garnet
highlights in color. According to Guinness, ââ¬ÅGuinness beer may
appear black, but it is actually a very dark shade of ruby.ââ¬Â
Opaque. A thick, creamy, long-lasting, tan- to brown-colored
head is characteristic when served on nitro, but donââ¬â¢t expect
the tight, creamy head on a bottled beer.
I know this is a snapshot of one type of each but confirms my thoughts on both styles that the porter is the weaker in body and colour whereas the stout may be creamier,thicker and darker.almost black. As far as flavours go I think they are much the same and I wouldn't be fussed if someone called it a porter and I thought it was a stout as long as it's good!"