You should use nitrogen. Air can bleed across the o ring seals in the reservoir to contaminate the oil. The nitrogen molecule is 4 times larger than oxygen and won't bleed through. Nitrogen will reduce the maintenance schedule.
The psi you use within the specified range will affect the ride and handling. Experiment and find out whet you like.
As others have mentioned; must be nitrogen and 150 psi is a good starting point. Although you fill them via a standard Schrader valve you need an inflator with a lossless chuck. This is because there is very little volume. In fact, even using the Penske inflator to check the pressure will drop the pressure by about 20 psi. I have a post about it here.