- ability to sandbox Google Play and Google Apps so that they live in their nice little Google bubble and have no control over my phone overall
- ability to run all applications sandboxed with fake permissions that I can whitelist for each application and without letting the app know it doesn't have the permissions it wants. Want location? Give the app a location point I've fixed for that app. (Or pass through real GPS location if I've chosen so.) Want contacts? Give the app empty contacts list. Or if I've allowed, give the app the contacts I've whitelisted.
The Android/Google ecosystem is all right in itself, I just want to limit all of it inside a cage that I control. I want the exact same for my browser: I want webpages to run in a highly controlled sandbox with my choice of spoofed environment and permissions instead of assuming any power over my system. Or my Linux desktop where I firejail or sandbox certain proprietary apps outside of my distro's repositories.
That said, I think the marketing of GrapheneOS could be better. Every introduction of GrapheneOS I've seen paints the image of Graphene being "Absolute security, no compromises", whereas in reality GrapheneOS is the most "Things need to work, no compromises. Then make the rest as safe as possible" custom ROM that I've used thus far (in particular regarding them allowing you to install Google Play, rather than using MicroG).
I understand that you can forego the final relock of the bootloader and install Magisk into Graphene. How that fares with future OTAs I do not know.
There are other differences that might impact you. The eBay app does not work on Graphene but is functional on Lineage. Graphene's launcher and keyboard are poor compared to Lineage. The Vanadium browser's dark mode is not as good as browsers on Lineage. Pattern lock is not available on Graphene.
Try them both, if you can.
Generally I think the marketing of GraphenOS is interesting. They are usually positioned as the "Absolute security. No compromises." ROM, but in my personal experience, they are the "It must work. No compromises. Then make the rest as safe as possible"-option, given aspects like using the "real" Google Play Services (if desired), but sandboxed, instead of MicroG or unrestricted Google Play, which pretty much all other ROMs roll with.