Is it better to use the mask or just the mouthpiece with the Philips Respironics InnoSpire Go nebulizer?

Its neither better nor worse. Just remember to inhale through your mouth when using the mouthpiece. When using the mask, it doesn't matter if you inhale with mouth or nose or both, just be sure that you have a good seal around the mask cushion. Also be sure that you have the Mask Adapter to connect to the nebulizer.