TheMadScientist - 5:14 am on Jan 7, 2013 (gmt 0)
I think swanny's advice is great ... To 'rephrase', I would say:
If you use an underscore currently, remember 'Cool URLs Don't Change', so it's likely best to leave it the way it is ... Moving forward, to create a new page, a hyphen is almost always better, so just use one.
The difference between the two, in my opinion, is really not worth a debate, because they're about the same either way as far as search engine treatment goes from what I've seen, so let's not make a big deal about what's essentially nothing ... If you're building a page on a new URL, use a hyphen. If you've built a page on a URL, leave it the way it is ... There's some things that are best kept simple, and this is one of them, in my opinion.