When was the Clear Blue test; had you been testing for pregnancy since the 27th of August, or was it more recently?
Had you taken a morning-after pill the literal morning after sex with the second guy, still turning up pregnant could have suggested the first guy was the dad. But taking it five days after sex with the second guy wouldn't affect a pregnancy from either guy. That means its failure to have the desired effect doesn't rule anyone out.
Looking at when your period came, if you were super regular, you would have been expected to ovulate around the 13th - 15th of August. But ovulation doesn't always happen two weeks after day 1 of the last period, and especially since you aren't super regular you unfortunately can't count on that to help you narrow things down.
Clear Blue giving you a number of weeks is merely telling you that the amount of hCG it detected in your urine was in line with 2-3 weeks since conception of the average pregnancy. This isn't going to be definitive for you, it's just based on the average hCG level of a lot of women. HCG varies a lot, not only in the weeks of pregnancy but even during the same day.
Your best bet right now is to get in immediately to see an ob/gyn and get an ultrasound, this week or next week at the latest. You'd have to hustle to find a doc who will do an ultrasound this early, but it matters because ultrasounds are only helpful for dating conception quite early in the pregnancy, and it would also be asking a lot of an ultrasound to try to help you determine the dad when it's only a 5-day difference between the men. But if an ultrasound is going to help, now is when it will. (Counting by your period, you're at 6 weeks now, not 4-5 weeks.) If you go in, ask them not to start the count of your pregnancy time period by your last period (which is the way they usually do it), tell them your cycles are irregular and you need to have the conception date figured only from the measurements of the baby. Or, to make it simpler, just tell them you can't remember when your last period came. Then they won't skew their results by typing in your last period and running everything from that date.
The reason you want an ultrasound so soon is that after not many weeks, embryos can diverge from the average growth rate, and some can grow faster and some slower than average. You want to grab this baby's size and developmental markers right now.
If you can't get in to see a doctor who will do an ultrasound at your first visit, and if you can't get in this week or the next, your next option will be to do a prenatal DNA test with Ravgen or the DDC. Since these are expensive tests, I do suggest trying the ultrasound route first. See also the FAQ's at the top of the DNA/Paternity community's threads for more info about this. Ideally, you would do a prenatal DNA test with both men.
You asked who is the more likely, and my answer is guy A. Maybe he did not pull out fast enough. ClearBlue has a neat advertising gimmick with its days of pregnancy, but hCG levels vary so dramatically that one should not use them with confidence. Your cycle suggests that 07/15 is closer to ovulation.
There are other indicators of when ovulation occurred. Female libido peaks at this time, as does vaginal discharge. If one encounter was more memorable than the other, focus on that.
Just an fyi for anyone- go to an abortion clinic if you want an ultrasound right away, they are usually free at planned parenthood if you have no insurance and they don’t do the procedure right away so you can walk in and they will measure the baby’s for you to see if you qualify for for an extraction yet. Not so much w a normal obgyn