Due date clash?

My last period started 10/26/12. It lasted maybe 3-4 days, and was really light. According to my LMP, my due date should be August 2nd, making me 4w3d on this past Tuesday, when I had my first ultrasound.

My ultrasound measured at 5w1d, so 5 days ahead, making my due date July 29. Which due date would be more accurate? I started having symptoms on 11/6.

The ultrasound

Ultrasound. That is what your doctor will go by anyway.