Graphene has a bunch of features that you may or may not agree contribute to privacy over all (funny since it only operates on Google's devices, that is, devices designed by a US enterprise).

LineageOS has some little QoL features but otherwise keeps it very AOSP. The most prominent difference is the dependence on specific device maintainers, meaning that some devices may get stuck on A13 while others may already have A14 or 15, though imo it's a small price to pay for having a modern OS on a 10 y/o phone or tablet. Graphene only provides support for Pixel models that have security updates, so you will end up with LOS if you'd like to keep using your Pixel 2/3/4 etc.

LOS can get rid of the pill at the bottom of the screen 😏