Both subplot options A and B could indeed add significant tension to the story, but if you have to choose one, I recommend going with A) one in which the character's decision has dramatically different effects on a dear friend in each reality.
Here’s why: This subplot creates an emotional connection and stakes that are personal and relatable. The character’s decision not only impacts their own life but also significantly affects someone they care about, which raises the tension and urgency surrounding their ultimate choice. The character will feel the weight of their decision in a more profound way, as they grapple with the potential consequences not just for themselves, but for their friend’s happiness and well-being.
As they navigate their two realities, the character might experience guilt or regret upon seeing how their choices directly alter their friend’s life in each scenario. This adds layers to the narrative and can enhance the emotional climax when the character realizes the full ramifications of their choice. The contrasting experiences of their friend can serve to heighten the stakes, making the decision point at the end even more pressing and complex.
While subplot B (the hints of a head injury) could add an intriguing layer of psychological tension, subplot A provides a more immediate and relatable conflict, making the character’s dilemma resonate on a deeper emotional level.