There are a few reasons why smiley is a better solution:
Webkit+Font Management software can mess up local references, like turning glyphs into A blocks. (crbug.com/33173)
On OS X, Font Management software may alter system settings to show a dialog when trying to access a local() font that's accessible outside of Library/Fonts. More detail on my bulletproof post. (crbug.com/29729) Font Explorer X is also known to mess up other stuff in Firefox: bugzil.la/531771
Although it's unlikely, you could reference a local() font which is completely different than what you think it is. (Typophile post on different fonts, same name) At the very least its a risk, and you're ceding control of the type to both the browser and host machine. This risk may not be worth the benefit of avoiding the font download.
These are all pretty edge case issues, but it's worth considering. FontSquirrel has already made the smiley syntax the new default in the Generator, and you should use it going forward as well.
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