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.
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.