pdagenius 'degoogling' isn't the priority for the project and is a misnomer. Happening to start from a position of no default connections to Google nor having their proprietary apps installed by defauly is a Bob Ross-ian Happy Accident for those of us looking to reduce and restrict our exposure to BigG.
Installing Play Services poses no additional security risk due to how GrapheneOS implements it.
Privacy wise however you are only ever as exposed as you choose to be based on what information or access you choose to provide to them. Using a personally identifiable account grants Google the ability to tie you to your usage of them, using GrapheneOS though limits that to when/how you use them unlike stock where they have access regardless.
Play Store is 'better' only in so far as you gain the benefits of its featureset. Aurora as a Play Store front end is the same source but without such features as unattended updates and auto updates etc.
Aurora is fine to use alongside Play Services if you dont want to use a personal account or cant create a throwaway due to required info when registering etc being an issue.