definitely march seventh.
To me, it doesn't really seem like either of those two fit your due date. If your due date is December 18, then your last menstrual period should have started around March 13th...and conception should have occurred about two weeks after that, so March 27th. BUT, your baby may be measuring a little big or small for your actual gestational age...so it's hard to say.
Honestly, I think it could be either...but if I were to guess, I would actually guess the one night stand (and that you just ovulated a little late that cycle). Do you know when you had your last period?