I would go for the refurb, but be cautious as some people have claimed that they received iPhones with minor physical damage (scratches, scuffs, etc.,).
If there's a problem or something is not working correctly within the 90-day warranty, you can take your newly received iPhone refurb to an Apple store and they will usually replace it with a new one for you.
Both of the iPhones I've owned have been refurbs and I never had a problem with either of them.
Regardless, I would wait until late June/early July for the new iPhone to be released. Last thing you want is to buy a "new" iPhone 3G and have it outdated in less than 2 months.