The biggest difference is that /e/ is one of the few OSes that has MicroG pre-installed, which allows apps to make use of some features of Google Play Services (e.g. push notifications, map widgets, and COVID-19 exposure notifications) through a FOSS client. MicroG lets users turn on/off some of these features, and does not support Google's APIs for ads or analytics.
LineageOS does not support the mechanism (signature spoofing) that enables MicroG to replace Google Play Services. The developers of LineageOS consider signature spoofing to be a security risk:
As of now, I've only used two Android forks, Cyanogenmod and LineageOS, primarily because they work on so many devices. LineageOS has a whole list of cool custom APIs that I've considered taking advantage of in some future project, but other than that I'm not sure.
FWIW, nowadays, thanks to Project Treble, most ROMs have GSI (Generic System Image) that will work on any device released with Android 8 or more recent, assuming OEM allows bootloader unlock.
/e/ is a fork of LineageOS with the goal to be usable by non-geeks. So it pre-includes a selection of FLOSS apps to make the device usable, like a Map app, cloud backups (well LineageOS now have it), calendar sync, etc.
