TL;DR at bottom
AJ's suppose to be more for lone wolfing and finding things on your own and fixing stuff and leaving. She also boasts the highest combined stamina and speed out of the repair trio which means she gets to locations quicker than the other two. She also has the highest stealth at 10 though this doesn't mean much when compared to Deborah as Deborah can also jog silently with 9 stealth but it does mean AJ can sprint quietly with a certain perk. AJ however, has the lowest luck in the game making her the worst in group play (as in grouping up and fighting Jason together as a team) because she will break melee weapons easily (the bat will most likely break after 1 or 2 swings) and will have to rely on long range weapons to help with against Jason which will typically leave her with one shot to help without finding another weapon. AJ has a 7 in repair, making her have the lowest repair stat of the repair stealth trio, but these skillchecks are not very difficult they are just shorter than Deborah and LaChappa's so having a 7 really isn't that bad as it allows her to have stats in other places. The last thing about AJ is her composure. 7 composure allows her to be alone longer and get so afraid as fast meaning she doesn't appear during sense as often as Deb or LaChappa. This lets her not get as punished as hard for being alone longer than others.
Deborah's suppose to be flexible between being by herself and being in a group, every now and then assisting at swinging at Jason if need be. Deborah has the same speed as AJ but has lower stamina which means she'll run as fast as AJ but for not as long. This makes Deborah more dependent on teammates in some situations compared to AJ (though the difference isn't that huge). She does have 9 stealth which means she can jog silently making her valuable in going solo, but cannot sprint silently with a certain perk. Deborah has a higher luck stat than AJ at 3 which may not seem like much but will make her more useful with the bat as she will get more value from it as it won't break as easily as it would with AJ. This makes Deborah better at group play because she can contribute to helping/protecting a teammate or an objective more often than AJ. Deborah is still dependent on long range weapons like AJ but not as much as she can still get value out of the baseball bat (which has a 100% stun iirc but breaks easily) due to her luck stat. Deborah has a 10 in repair which makes repairing a breeze, it is easily her best stat but keep in mind that having a 7, 8, and 9 are also good, having this high a repair stat can actually hurt her in other areas, as repairing with the said repair stats from before is not difficult. Having a 10 does make it practically impossible to mess up on repairs though, so there's that. Deborah's composure at 5 means she has decent composure and can be alone for awhile but not as long as AJ. This allows Deborah to tread between staying alone and meeting up with a group.
LaChappa (I know you didn't really ask about him but he's the 3rd of the repair stealth trio, so I'll mention him anyway) is suppose to be the group player Repairer. He stays with a group because he needs people to protect him. This is due to his extremely abysmal stamina and speed stats at 2 meaning he will almost always depend on a teammate to help bail him out of a bad situation should Jason find him. To add on that, LaChappa has the lowest stealth of the trio, at 8. This means LaChappa cannot jog silently but will make much smaller "white circles" when he does compared to others with worse stealth stats. He has the highest luck of the repair trio with a 5, meaning his weapon is less likely to break than the others making him better in combat situations and supporting an ally would got grabbed more often. This gives him a better use in groups compared to both AJ and Deborah as he can help with teammates more often in group play as his weapon is less likely to break, making the baseball bat a good choice on him. LaChappa has the same repair stat as Deborah. Refer back to Deborah's section of repair. LaChappa's composure is the lowest of the trio, at 4. While this is not super terrible, its lower than Deborah's which means he will freak out more than she will and faster too. LaChapp will benefit the most out of the trio from being with others because being near other counselors keeps one's composure in check.
AJ is usually best as a lonewolf due to her: Good Repair, Decent Speed, Decent Stamina, Top Notch Stealth, Worst Luck, Good Composure
Deborah is best as a flex pick, she can work with a group if she wants or she can work alone due to her: Top Notch Repair, Decent speed, Pretty Good Stealth, "Meh" luck, Decent Composure
Lachappa is best in group play where he can help his team fight off Jason around objectives due to his: Top Notch Repair, Bad Speed, Bad Stamina, Good Stealth, Decent Luck, Below Average Composure
THAT BEING SAID
AJ is probably best in random games due to the fact she's really independent, Deborah treads slightly behind her because of her lower stamina and the fact she can't lone wolf as well as AJ.
Don't think the difference is that big though, tbh.