Which is better erythrocytopheresis or simple blood transfusion for the prevention of childhood stroke?

Pheresis. If the risk for stroke is due to increased red cell mass in the blood, which makes the blood thicker, then removing just the red cells by apheresis is an option. Please refer to a children's hospital hematology department for more specific answer.
Maybe neither. If the child has a blood disorder that caused the stroke, then one or the other may be appropriate--or neither. If the stroke is unrelated to a blood clotting disorder, then cases require therapies dependent upon causation, if one can be determined--sometimes causation is elusive in children and in adults.