Preferred incision is vertical after vertical scar so as to give minimum scar to patient. but after one cesarean section one can give transverse incision as less adhesion are found in the lower area in the abdominal wall.
I would offer the patient the option of using the same scar or using a transverse suprapubic incision which, although a second incision, heals better and faster and therefore accelerates post-op recovery.
it depends on the appearance of the previous scar, if it is hypertrophic and with keloid, better to excise, if it is not cosmetically unpleasant, transverse does have definite advantage in terms of healing, avoiding complication due to adhesion, cosmetic as well as post op discomfort. Explain in detail to the patient and she can make the decision.
If there is already a vertical scar on the abdomen then i would go for a repeat vertical for cosmetic reasons. In addition there could be adhesions from the previous incision and negotiating those through a pfannenstiel or Joel cohen's could be very challenging.