Personally, I would buy. That means the fare is around $400 plus taxes. With United sending smaller planes and no longer offering daily flights out of EWR, the supply/demand curve favors the airline. Waiting may save you $50, but could also mean the fare jumping up $100 if enough people buy at $518. IMO it is a lot of risk to wait for little reward. For a reference point, the cheapest fare I have found on United nonstop out of DC this year is $620.