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.
Lineage covers many devices but is not very privacy friendly. It does not include Google integration by default. The ßain advantage is longer software support.
Graphene is most secure and privacy friendly OS and comes with support for sandboxed GApps (optional). Unfortunately it is only made for Google Pixel devices so I'm out.
Depending on your reasons and device you could also check alternatives like /e/.
I use graphene because I just like its way stricter security features, such as randomized numbers on the keypad when unlocking and it not having any Google services by default
But if you dont care about that and you would rather use MicroG (which is a little project that still runs at root access, but dosent send as much data to google) instead of G services just being sandboxed you can just as well use linaege
Also, linaege is a Team of volunteers that maintain many phones, as grapheneOS only makes Roms for the Google Pixel Series, so all their effort is towards the pixel phones. But as far as I know Graphene only maintains the Roms for phones that are still maintained by Google themselves, only giving a year or so extended support, but I dont know how long the Maintainer(s) that support the Pixels will maintain the Rom