Anyway, let's set the stage (based on a true story).
You're sitting with a friend in the comfort of your own home, which is great, but that means you can't have Guinness from the tap. You open your fridge to grab refreshment for your guest and yourself, and there before you sits a bottle of the dark, delicious nectar. Beside it, you spot a can. Being a good host, you ask them which they prefer. They say either is fine, take your pick. Now it's up to what you'd rather drink. Which do you choose?
I know my answer, but it's no fun if I just come out and say it from the start. Instead, let's look at the key differences between the two. First off, it's worth pointing out that conventionally the bottle is the "right" choice with most beers. The glass makes for a nice, neutral storage material that doesn't notably color the flavor of the beverage inside. Cans, on the other hand, can add a bit of a metallic taste to beer, especially if the cans have been sitting for a while. Aluminum, while a useful substance, is not noted for its flavor, so this tendency can be a bit of a problem. Admittedly the flavor profile of a dark beer like Guinness does a better job of hiding this slight tainting than, say, a lager does. Nonetheless, it's still there and it's still detectable.
The can has its perks, though. First off, it's bigger (14.9 fl oz to 11.2 fl oz) and more beer is better. This is only true on a one-to-one basis, of course, since bottles come in six-packs while cans only come in four-packs, so overall that's not a deciding factor. Neither is the fact that cans don't require a bottle opener, since I'll assume that most of us have bottle openers in our homes. So neither of these apparent advantages really counts for much of anything.
That means bottles are better, right?
Wrong! (Of course it's wrong. That's how that setup always goes.)
The really pivotal difference between the two is the method of carbonation. It has a significant impact on the overall flavor. Bottles come pre-carbonated, which is sort of okay, but even when you first open a bottle it never has that full, rich, chocolatey flavor that only a proper level of fizz can bring out in the drink. And when you leave an opened bottle sitting for any period of time, it starts going flat, so you need to drink it quick or it starts to taste worse. On the other hand, there is a little widget that rattles around amusingly in the bottom of the cans. Their use of this widget kicks up the levels of creaminess (I know David Mitchell hates that word to describe beer, but I don't care) so that the first sip is vastly more complete than anything you'll taste from the bottle. Further, that funny little ball keeps working its bubbly magic all the way to the bottom, even if you leave the can sitting for a little while, so there's no real danger of your drink getting less pleasant towards the end.
Final conclusion: quite unlike most beers, Guinness is better from the can than from the bottle. The widget, as opposed to normal carbonation, enhances the flavor in a way which far outweighs the hint of aluminum tang from the can.