Both Manjaro and EndeavourOS are based on Arch, but Manjaro has its own repos, while EndeavorOS uses the Arch repos (apart from having an additional one for some useful utilities, scripts and some basic theming). This is a big difference.
On Manjaro, the repos are built from the Arch repos, but the Manjaro team modifies many packages, like kernels for example which are custom built for Manjaro (a lot of packages in the Manjaro repos contain customised settings and Manjaro-specific theming). A big difference is in updates: Manjaro has its own stable and testing branches which delay updates. For example, on Manjaro stable the updates are pushed maybe once in a couple of weeks, and they are huge, often several gigabytes.
On EndeavourOS you directly download packages from Arch mirrors, there is no middleman. You are basically running a pure Arch system. The updates are similar to Manjaro unstable branch, very frequent and small: there are new updates every day, sometimes several times a day. Of course, you don’t have to update straight away, you have full control over updates.
On EndeavourOS, just like on vanilla Arch, there is no MHWD, so you have to know which drivers you need. For older or more obscure hardware, the drivers might not be in the Arch repos, so you have to install them from the AUR. The same is true for older kernels: for example, if you want to run kernel 4.19, you have to install it from the AUR. On the other hand, unlike Arch, EndeavourOS has an additional repo with a handful of great scripts and utilities that can help you a lot in this regard. This additional repo also has yay so you don’t have to build it manually. Instead of Manjaro’s pacman-mirrors script, you use reflector (which is much better, actually) or you have to manually configure the mirrors.
The default install of EndeavourOS is much cleaner than Manjaro’s default install, though both can be customised, of course. Out of the box, EndeavourOS is very lean, has almost no bloat at all (no snapd, no Steam, no FreeOffice garbage), and very little theming (except for the offline Xfce install, if I’m not mistaken). It also does not do stuff behind your back (for example, there is no annoying pamac-timer service that automatically resets your mirror config every week, like on Manjaro).
What else? There is this forum, which is probably the best thing about EndeavourOS, a very friendly place with many users willing to help you. 