I made the Dry Irish Stout kit last year, and side-by-side taste test with bottled Guinness it was nearly identical. (Everyone preferred the NB kit though.) Dead-easy kit too. I sort of think that Guinness tends to taste thin and watery, so a copy of it would be the same. I try to respect it as a style, just maybe not my favorite.
I just did the St. Paul Porter and so far it has been excellent. Lots of body, lots of flavor. It seems like it could compete with some of the benchmark straight-Porter styles out there (like Edmund Fitzgerald). To me SPP is much more exciting than the stout. But if you’re dead-set on Guinness the stout kit will give you that.