Well if you were charting and you KNOW when you ovulated and approximently when the egg implanted than go with that.
Remember that date of conception is NOT the day you had sex. Sperm can live inside you fro up to 7 days.
If you don't know for sure when you ovulated go by LMP. Due dates aren't that big of a deal anyway. Only 5 percent of babies are born on their due dates and your baby can come any time between 37 and 42 weeks.