The rationalist approach suggests that there is one right answer to strategy and that the task for businesses is to find it, or at least get as close to it as possible. Anyone can find this approach so the CEO may delegate to others to help find it.
As you would imagine the incrementalist approach is different in that it focuses on the ability of the firm to re-adjust it's stated objectives in the light of new knowledge or understanding.
The rationalist approach is better suited to projects as it uses fixed resources to get towards a future desired state. The incrementalist approach is more flexible allowing for changes in the desired future state as well as resources allocated to getting there.