Before this test I considered the differences negligible, without really knowing what the differences are. Now I consider Helvetica much more elegant and incisive.
Lately we've developed this fetish for sites being pixel-perfect across any display.
The web was designed to decouple the way a site was designed from the way it's displayed. To allow it to reflow, and people to use their own designs.
If you really want pixel-perfect layout, why not just use bitmaps and imagemaps ;)
Although, a common way to get them out of their "I know better" state is to point out the time factor and how much extra it will cost them. Suddenly they will worry over browser incompatibilities all day long.
The worse are the people that point out that the fifth paragraph on the third page ends with three words on the final line in the mockup but the website version has four words in Firefox and two words in Chrome. I just want to so tell them to just let the text flow its own way, dude. In the end I tell them in a nice way they're being silly.
Note that I never said that it shouldn't be decoupled, or that people shouldn't use their own designs. But if you want people to see your site as you intended, I'd for sure want them to see it with the exact font I choose, not some close alternative.
I've always taken the view that you throw the text at an engine and let it do the rendering. If it doesn't look right, the engine needs improving. Manual tweaks fall into the realm of premature optimisation, as chances are the content will change faster than the tweaks can be made.
I can see that beautiful typography is nice to have, but apart from automation, I can't see a way to apply it on a mass scale. Crafted typography is the typographic equivalent of assembly language?
The appearance does matter more for display text (logos, headings, etc.), but you're using font-face for those anyway.
Alternatively, @font-face is supported by just about everything at this point, opening up the opportunity to use much better fonts, like open sans
(also, helvetica is a print font. It doesn't work for body copy at the sizes commonly used for screen text.)
On Windows, however, Helvetica isn't (at least from my experience) as nicely hinted, or at least doesn't play quite as well with Microsoft ClearType. And that's if it's installed, which is less likely on Windows. In that case, if your font stack specifies a fallback to Arial, which is ClearType's best friend, you'll probably be fine.