Neither of those is a privacy or security hardened OS. AOSP is much more private and secure than the desktop Linux software stack and GrapheneOS greatly improves upon it. GrapheneOS does not promote Google services. /e/ always gives privileged access to Google apps and services and ships with a bunch of Google services active default. GrapheneOS doesn't come with sandboxed Google Play and doesn't connect to Google servers by default.

https://eylenburg.github.io/android_comparison.htm

https://grapheneos.org/features

We have a non-exclusive partnership with Motorola Mobility to bring GrapheneOS to their next generation and later devices. This is necessary because non-Pixel devices don't provide the updates or hardware-based security features we need. GrapheneOS supports the latest available Pixels and plans to add support for future Pixels alongside our partnership with Motorola, which is not exclusive and does not preclude having additional OEM partnerships.

/e/ greatly reduces privacy and security compared to the standard Android Open Source Project. /e/ has repeatedly claimed hardened devices mainly benefit criminals and pedophiles and have falsely claimed GrapheneOS is mainly used by criminals on multiple occasions too. That's very relevant to the context leading to someone creating this thread.

https://grapheneos.social/@GrapheneOS/116353973732143171

You don't have to install Google Play Services at all. If you have to install it anyway, then Graphene's implementation seems a lot more mature than microG or any other solution out there for /e/ OS.