By my doc and by my lmp I'm due Dec 7; by US I'm due Dec. 1st. When I asked my Doc to change my due date, she said their typical protocol is to go by the last period, unless the US due date is off by more than 2 weeks. So technically my due date is the Dec. 7th, but I usually tell people I'm due the 1st.:) So I say the US is more accurate, but guess it's up to you what you tell people; ).
Go by the ultrasound, thats always the most accurate :)