Here's something interesting to do with the same test. Instead of guessing which one you think is Helvetica, choose the one you like better. Then see if Helvetica shows up more times than Arial (or vice versa).
Great idea! Personally I had no bias towards either font, but now discovered that I do in fact like Helvetica a bit more (12 out of 20 times). Would be interesting to be able to see a test like that with statistics on how many chose each font for each logo.
Yeah, going into it I didn't know much about the differences between the fonts but was curious anyway. After getting a few wrong I switched to choosing which ones I thought looked nicer and started getting Helvetica more times. That reminds me I need to finish watching the Helvetica documentary.