How do I describe a person who is consistently attentive?

Marathoner and sprinter seem like the most likely terms to me, especially if you are contrasting the two styles. Many other words describing people have connotations beyond just how long they prefer their projects to last on average.

You could focus on the qualities of the projects they like to work on. One likes short-term projects, and changing situations. Adaptable, flexible, able to roll with the punches, change tolerant come to mind here as positive qualities the sprinter might possess.

The marathoner likes long-term projects, which means larger, more complex projects. Focused, sees the big picture, strategic planner come to mind here as positive qualities the marathoner might possess.