The blades on the impellor are little wings. When facing the fan you want them to produce max lift at min drag. This would be very high pitch. To win this race make sure you make or get a good estimate of the air speed of the fan and your expected impellor speed so you can calculate a good velocity vector. That will allow you to optimize the blade pitch for max lift and min drag. Sence you are using the impellor to drive wheels you want max torque out of it not max speed.

I would turn the impellor in the same direction as fan.

It would take quite an analysis to determine which is truely better or if there is a real difference.

Denis