Google’s ranking is highly complex and something like the - vs _ in a URL is such a very very small fraction of the puzzle its not really worth fretting over, really.
I think you give Google too much credit. 😉
I don’t really know either way because nobody here really knows how Google works. You can argue either way about URL’s but the bottom line is you probably aren’t going to get a first page ranking on a keyword which solely exists in your URL.
I would say the Matt Cutts blog post gives me the teensy weensy, itsy bitsy push I need to use dashes rather than underscores. I don’t buy the argument on URL readability either. URL readability went out the window a long time ago for me. There are just too many situations where the URL is used to shuttle non human friendly information for me to even take the effort to raise my eyes enough to look.
Unless I see something clearly stated in the Google webmaster guidelines, then I would stick with the Matt Cutts article.
Those search examples with expression_engine vs. expression-engine are faulty, as that’s the wrong side of the coin. The point isn’t about how Google treats dashes vs. underscores in search terms but in the URL.
Sure, but what if Google treats them the same in the URL or in the content. Maybe this sounds stupid and Google should be better but I don’t have as much faith in the intelligence of Google as everyone else does. Google search is still just a crappy web app just like anything else out there and search is still in its infancy.
If you guys want to keep your searches strictly to URL’s then why not use the URL operator?
Try doing a search for this…
inurl:asdf-asdf
vs this…
inurl:asdf_asdf
Notice that for “inurl:asdf-asdf” you get the following…
asdf+asdf
asdf/asdf
asdfasdf
asdf-asdf
If you do a search for “inurl:asdf asdf” (with the space) then you also get results for asdf-asdf.
But if you do a search for inurl:asdf_asdf then all you get is the underscore versions.
My conclusion…
Google is stupid. You all put too much faith into them. IF (IF!!) you lived in a world where you could only rely on your URL’s for search results and you want to capture the widest audience then use dashes.
EDIT: Though I do agree that the URL is not as important as the content. As Nevin says, it is just another piece in the puzzle.