Camera quality is the same within the same app on either.
Pixel Camera can be used on GrapheneOS with full features and photo/video quality if you want it.
GrapheneOS Camera has support for HDR+ on Pixels for regular photos and has Night mode too. It has EIS and HDRnet for video recording. It has a single exposure slider rather than their dual exposure sliders. It uses each of the cameras via zoom level / light level in the same way. More advanced features and configuration are being added to it over time.
Pixel Camera has more features and the HDR+ it uses is more aggressive which makes the photos look higher contrast than a more natural look.