Here's the two side by side. Helvetica's curves are very rounded, and similar for many different characters, too much consistency. SF has less consistency throughout the set to make the individual letters more identifiable. Also, Helvetica has very wide characters, so there's little space between them. SF has narrow characters, and can have a little more space between them even though the word is the same width. When the letters are stacked too close together it could be harder to read.
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Thanks for this, I was hoping for a direct comparison.