Hi, I assume you are not going to do paternity testing until after the baby is born, since the due date is getting so close now? If so, are you going to do the kind of test admissible in court to prove paternity, or are you just going to do a drugstore test that they sell for people to satisfy their curiosity? I ask because you really should be getting the kind that is able to satisfy the court.
Here's the thing, In your description of the situation, you don't give enough information to be able to suggest to you which guy is the dad. For example, you said your first ultrasound gave you a due date of December 27, but don't say how early in the pregnancy the ultrasound was. (It makes a difference. The later in the pregnancy the ultrasound was, the more inaccurate it can be about the due date.) And, you also say that the second guy is more likely the dad because you had sex around cycle day 13-14, but don't say how regular your menstrual months are. (Ovulation doesn't come automatically two weeks after the first day of your last period, it comes two weeks before the first day of your *next* period. If your menstrual months vary in length, ovulation can come earlier or later than day 13/14.)
In answer to your question about condom use, if the condom had broken, you wouldn't have felt it because body-temperature liquids are not noticeable in a vagina, but you or especially he would have seen it. That said, unfortunately condoms are only about 80% effective in use overall, for various reasons including not being put on early enough, and getting pre-ejaculate on the outside.
In general, if someone has sex unprotected at the beginning of her fertile window and at the end of her fertile window, the first one will have the edge. But since the sex at the beginning of your fertile window was protected (or at least 80% protected), and since there was lots more sex at the end of your fertile window and it was not protected, it probably means that the second guy has the edge. This would be especially true if your periods come every 28 days, month in month out, and never vary.
But, especially if you aren't married (and your child therefore needs legal proof of paternity to be in his records), you should do DNA testing with both guys. And use the kind of test that is admissible in court. If you want to start with your preferred guy and then only test with the other if the first guy comes up negative, that would be a way to save money (given that the first guy comes up as the dad).