Having used LineageOS myself for about a year (before I even thought to ask questions about privacy or security), my own view is that its focus is generally to provide features and OS updates to as many phones as possible, including old EOL phones, whereas GrapheneOS' focus is to provide enhanced privacy and security to its users.
LOS feels like a novelty (e.g. having Android 14 on a pixel 3a) while GOS feels purpose built and by choosing GOS over LOS you really aren't missing out on features- just some additional status bar customizations/functionality and screenshot features. In fact, in my opinion I get more features out of my phone by using GrapheneOS than LineageOS.
Control is a big one for me too. GOS provides a lot of control that LOS (or any other phone in existence for that matter) just will never provide.
Also, GOS provides superior documentation, accessible support forums and community which I have found super helpful.
Side note: LineageOS was also more involved and more nerve racking to install and sometimes a pain to update.