Guy 1 has the edge over guy 2, but unfortunately it is too close to call from your ultrasound since it was in your tenth week. All it would take to put guy 2 firmly in the running is for the information from the ultrasound to be 4 days off, and in your tenth week there is a margin for error of about that many days when using an ultrasound to make this kind of determination. In your shoes, I would assume guy 1 has the better chance but plan on a DNA test once the baby comes.