These two possible fathers are too close together in time to make any prediction, Rosie.
If it's important, you can have a paternity test done while you're pregnant for around $1500, or much less expensive once the baby arrives.
Best wishes.
Unfortunately, although you did have an ultrasound that helped you pinpoint it as much as an ultrasound can, when the sex is only four days apart there is no way to be totally sure. Your scan was early enough for it to be useful but seriously, it would have to be spot on, and at 7w4d, it could be off by one or two days. And you need there to be no margin for error, to be totally sure.
If the ultrasound is correct in naming March 14 as the day of conception, then the first guy is the dad and you were already pregnant when you had sex with the second guy. Two days pregnant is still pregnant.
If you have the money, you can get a DNA test done before the baby is born. Ravgen and the DDC do them, it's a blood draw from you and cheek swabs from the potential fathers. (In a perfect world, you and the two guys would split the expense three ways, and then you would all know now instead of having to wait.) But DNA testing before the baby is born is ten times more expensive than after the baby comes, and for that reason (plus some women don't want both guys to know there is a reason to test), not everyone tests before the baby comes. You definitely need to test with both men after the baby comes; find out from your doctor how to get the tests done while you're still in the hospital.