I have to agree with all the posters who are advising you to miss the IAD connection. Those UA Express flights are terribly unreliable when the slightest bit of weather crops up. Besides IAD being just a horrible airport to transit through, I just don't think it would be a risk worth taking. The thing going for JetBlue is that that JFK operates with far fewer delays due to longer runways etc, than LGA, although at 700am in the morning you have the advantage of no built up delays. Good luck with your decision!
Adam