I had my 12w ultrasound last week. It confirmed that the second ultrasound date was accurate.
I agree, go with the earlier one. All babies grow at different rates. It's not uncommon for an ultrasound to be off a week or so either way. Besides, due date calculation is not an exact science. I've always gone by my first ultrasound date.
My doctors with both of my kids gave me u/s dates that were sooner than the lmp dates. I ended up having my son waaay after the u/s date on the 20th (I had to be induced), the u/s date was the 8th and the lmp date was the 10th.
The u/s date for my daughter was the 3rd, the lmp date was the 8th and I was sure of the conception date. She was born on the 8th.
Trust the 6 w 3 d ultrasound over the later one(s). Babies do grow at different rates.