It could be either guy. If you count from your last period, and if your cycles are like clockwork exactly 28 days apart each time with no variation, you would have ovulated on April 6, when both guys' sperm was potentially in your body. (There is no guarantee that the guy who did not ejaculate didn't have some sperm in his urethra from a recent, prior ejaculation.) It would be a better idea to get an ultrasound right away and ask them to tell you the estimated due date, and to use a conception calculator to confirm when the conception probably was. Counting forward from periods is not as accurate.
If you need to know before the baby is born, DNA tests are available (contact Ravgen or the DDC) that can be done with a blood draw from your arm and a swab from the guys. The fact that you say you are having trouble deciding what to do with the pregnancy makes it sound like if it were one guy you would not want to keep the baby but if it were the other guy you would. If it is this important, be sure (if you go with a prenatal DNA test) to test BOTH guys. You want to get to one guy a definite positive and the other a negative.