A very big congratulations!
Personally, I would go with the 7 week scan. The week extra waiting will be better for you (less of a worry) than a week extra of waiting with the added burden of also hoping for a heartbeat in week 7 if you didn't hear it in week 6. Do you get what I mean? i had one at week 7 but didn't have the choice of week 6. The mental anguish is lessened if you wait a week!