Comparing just CalyxOS to /e/, both include microG as a substitute for Google Play Services. That's the main feature they have in common.
Some of their differences:
- CalyxOS's upstream is AOSP, while /e/'s upstream is LineageOS.
- CalyxOS is intended to be used with a locked bootloader, while /e/ is mostly intended to be used with an unlocked bootloader. (As an exception, /e/ supports a locked bootloader for the Fairphone 3, but I'm not sure if there are any other models that work the same way. /e/ install instructions for Fairphone 3: https://doc.e.foundation/devices/FP3/install)
- CalyxOS supports a few devices, since the project only considers devices that support relocking the bootloader with a custom key and have a monthly security update schedule from the manufacturer. /e/ supports a much larger number of devices.
- /e/ integrates its optional open source cloud service, ecloud, which includes email, calendar, contact, photo, and file hosting. CalyxOS does not offer an equivalent first-party service.
I've been really happy with /e/ in daily usage but I'm curious to see what other projects are out there.