I decided to try two Draught-style Irish Stouts, neither of them Guinness, in a side-by-side comparison. This sort of thing can be instructive, but in the case served mostly to remind me how limited my beer vocabulary is at this point.
Both beers look the same: rich, dark brown in color with thick (though artificial looking) heads. Beamish seems to have a slightly more roasted aroma, but it's not a large difference. Even differences in taste aren't huge, although I would notice if I grabbed the wrong glass. I think Beamish has a more nuanced roasted coffee/chocolate aspect. Murphy's seems to have a "lower" taste, metaphorically speaking, sort of like having more bass than treble notes. Also, Murphy's seems to finish with slightly more bitterness. Overall, I prefer Beamish. I can't compare either to Guinness, which I've never had from a Nitro can, and actually haven't drunk in a long time, now that I think of it.
I'm not sure what to think about Nitro cans themselves. They pour a beer that certainly looks nice. I normally love smooth, creamy, low carbonation beers, but these come across as more watery than rich. I found an old comment thread at Lew Bryson's blog which includes this helpful comment from John G., to which I'll add that I'm not sure if these stouts really do have enough "bite" on their own:
Here's the thing. CO2 delivers a "sting" when you drink it (literally from carbonic acid). In a Dry Irish Stout, you have a beer with a significant bite from both roasted barley (traditionally 10% of the grist), and from a high bittering rate (30-45 IBUs), combined with a low gravity body. One can argue, with that much "bite" already present in the beer, that any significant CO2 bite would be "too much" - hence the smoothing effect of the nitro can be seen as beneficial. However, in a hoppy beer, since you don't have the effervesence to bring the volatile hop notes to your nose, you are kind of wasting the point of the beer and supressing hop aroma with the nitro.