There is a disagreement between the Graphene and CalyxOS community about which is more secure/private: Graphene's sandboxed Google play store, or CalyxOS's MicroG. I've read posts advocating for both sides, but I don't have the expertise to have an opinion, and I decided that I don't want either software on my phone, since I don't want to run google code or play store apps.
Although I'm not expert enough to validate the following claims, here's what I've read.
Graphene people claim that MicroG needs elevated privileges to run, privileges that Graphene doesn't grant to any app. MicroG also loads and runs Google code (in a context where that Google code would presumably have access to those elevated privileges). Graphene's version of the play store emulates some APIs without using Google code (for privacy), and sandboxes the Google code that it does run, running it with reduced privileges. This is a security first posture, keeping in mind that if you don't have security then you can lose privacy via exploits of your security holes.
CalyxOS's MicroG emulates a larger fraction of the google play APIs, making it less reliant on google code to operate, and this is the source of the claim that MicroG offers more privacy.