The hyphen is still better, as I see it, but by a slim margin.
Google has traditionally considered the underscore as a true character and not a separator. They did this so people can directly search on technical keywords that contain the underscore character, such as _borders or mod_rewrite.
Google has done some fancy dancing in more recent times, in order to use the relevance signals that are present in underscore separated file names. However the hyphen still seems to have a slight edge. It's not enough of a difference to bother changing established urls, but if you're creating a new file, I see hyphens as the way to go.
By the way, don't be misled by the way words are presented in bold on the final SERP. That's a simple character-match routine, run after the SERP is generated for a query. It does not indicate that the bolded word was actually used in the scoring.