Those dates were within 5 days of eachother, so it's possible that either one could have resulted in pregnancy. You next ultrasound may be able to give you a more accurate date based on gestational size and age. You may need a DNA test to confirm paternity.
It's possible to get pregnant from pre-ejaculate, but it would be more difficult in the circumstances you describe than if you had not been using a condom. On the other hand, if the condom was in your system for 24 hours, maybe not so difficult. (I'm somewhat puzzled by this story in that neither of you noticed that the condom was not around after it slipped -- where did you think it had gone, if it wasn't in the bed?) That said, pre-ejaculate only has sperm in it if the man had ejaculated beforehand relatively close to the time of intercourse. This reduces the chances of him being the father compared to someone who had unprotected sex with you and ejaculated.
The ultrasound to use to parse the short timelines between the guys should be the very first one you had. Estimated due dates often vary over the course of a pregnancy because babies grow at different rates, but all embryos start out with the same developmental markers in the early stages. Did you happen to have an early ultrasound, such as in your 7th week or close to it? What did the doctor or ultrasonographer give you as the gestational age and estimated due date when you had the first one?