Yes, the differences are easy to see, as everyone has said.
But the important part is that there's a difference in feel and theme that's not really measurable and identifiable in direct comparison.
The subtle difference is far more important than trying to identify the tiny details that don't really matter. And in that sense, this game (while fun and interesting) misses the point.
It's entirely measurable. Helvetica has slightly heavier strokes, which are finished at right angles or closer to that than Arial, and prefers verticals and horizontals in general compared to Arial.
I've always heard the Helvetica snobbery (and haven't used Windows much since the 90s) so I was a little surprised to find that in most cases, I found the Arial logo more attractive. Helvetica has a little more "heavy-handed" feel to it which worked for short words in all-caps, but I thought the lighter look of Arial worked better for the rest.
Horses for courses, but you're in the minority. Helvetica is pretty widely considered a more aesthetically appealing font, especially amongst those who have a lot of points of reference.
It's like art: you develop more sophisticated tastes as you're exposed to better things. It's hard to say this without sounding snobbish, but if all you've seen is Arial, you'll find Arial familiar and comfortable. But the more time you spend looking at good typography, the more Arial will start to hurt your eyes.
Hypothesis: The difference in feeling that some people get from fonts is like the difference in feeling many people get when they pay a lot for the wine they drink. It's subconscious. It feels like a real feeling. It just isn't driven by the actual experience of the product.
In the wine world, it is possible to isolate this effect using blind taste tests. In the font world, there seems to be no robust way to show a font expert Helvetica without letting them recognize that it's Helvetica.
I don't think that analogy stands. You can tell there's something different between them, specially on a full page of text, without knowing what typeface it is at all. Helvetica is slightly more elegant. That said, it is no surprise that it's hard to tell the difference since one is a rip-off of the other.
Or like Stradivarius in classical music. There have been blind tests when a contemporary violin was mistaken for the Strad. Yet classical musicians still covet the Strad - my theory being that it makes people listen with this subconscious effect you've been talking about. Same sound, but if we believe it comes from a Strad, it seems better.
But the important part is that there's a difference in feel and theme that's not really measurable and identifiable in direct comparison.
The subtle difference is far more important than trying to identify the tiny details that don't really matter. And in that sense, this game (while fun and interesting) misses the point.