Automatic. As I'm using the DS4, I need the triggers for Accel/Brake. Can't use the sticks, never have been able to adjust to them. X and O are completely useless for modern driving games now they've reverted back to on/off.
Also, I'm not losing time driving with this setup on auto... so I can just enjoy the drive without having to shift, which suits me down to the ground with arcade racers.
Of course, if I'm on a racer that's more simulation oriented, I would be manual every day of the week. It is a problem with the DS4 though. I would be using a wheel for simulations though. T300 arrives next week, ready for PCars!