It could be either guy.
If you had known you were pregnant and had an early ultrasound, such as in the 7th week, and that early ultrasound had said you were due October 20, you might have been able to say that the first guy was more likely to be the dad than the second guy. But by week 34, using ultrasound data to try to guess at a conception date has a margin for error of +/- three weeks either direction. That means there is no way to accurately extrapolate which guy is the dad from an ultrasound by that point in pregnancy unless the sex with them was much more widely spaced in time. I assume the baby is born by now, why don't you do a DNA test with both men?