For me this is an eternal question. I love the impact of a single image. How conceptually you can say everything within one image, telling a story but also letting it open for interpretation.
On the other hand, series show different angles of the same subject and offer even more of a narrative. They also show a client you can be consistent with a style and offer variations on a theme.
I guess each project calls for its own pace, but sometimes I hesitate between the two options…