You need to do both to get the sex scenes, but I think helping Leif was better since it gets them on your side and you also end up saving/rescuing someone, while Deryl's path seems to have the same outcome whether you help him or not.
The fact that you don't remember who I'm talking about is a good reason to think it's dumb. Yes, that's who it is. If you spare him, you force him to pay the medical bills for Lucy's daughter, which allows for proper treatment of her Tay Sachs Disease. This, in turn, allows her body to be strong enough to survive.
I saved the daughter of Lucy, the blonde secretary from the beginning of the game on Liz's path. She shows up again in the shelter, but she says she lost her daughter during the attack (If you promised to help her earlier in the game). Daughter ends up becoming a new super human and you save her with Leif's help.
If you force Erik to pay Lucy more money and (IDK if this is a check or not) tell her daughter, "That's what hero's do." she gets excited & wants to join HERO, then she awakens as a super and Leif runs her into the barrier before the gas can finish her off. Everyone else dies, who didn't already leave the city, including Lucy (RIP) (convicts I think survive since they left before the gas started).
If you go the other path with Deryl, you still end up saving the convicts & they help Deryl, & you get to scan Deryl.