Graphene only works on Pixel phones. Graphene is more private and secure, but might be too many issues for people who do not care about such things that much. Lineage has better support for different phone models, and you can make it just like a normal Android OS, that just happens to provide updates for your phone years after the manufacturer stops sending them.

Neither of them is better than the other, it's just about your priorities. Get Graphene if you have a Pixel and you value privacy and are willing to tinker with it a bit if some apps don't work.