If you want fresh KDE, you can’t go wrong with either Arch or Fedora, with Arch being much fresher.
Keep in mind that with any Arch-based system, you will not be able to use Discover to its full potential (because it will break your install). On Arch, KDE Plasma comes without Discover (but you can install it manually). I’m not sure how well it works on Fedora, I haven’t used it in ages.
They are quite different. While they are both based on Arch Linux, all similarities end there. I’ve been using both for several years and my experience has been very different.
EndeavourOS is basically a pre-configured Arch Linux with a GUI installer. It uses vanilla Arch repositories for all of its software (except for a tiny additional repo for utilities and theming). On Arch (and thus on EndeavourOS) KDE Plasma comes completely vanilla out of the box, without any custom theming or customisation.
Manjaro uses its own repositories which are delayed compared to the Arch ones, so you don’t get the bleeding edge releases. In theory this is supposed to make Manjaro less prone to breaking, since all new updates get to be used by the Arch users before begin pushed to Manjaro users. However, in practice, and in my experience, Manjaro tends to break more than pure Arch or EndeavourOS, especially if you are using software from the AUR, which expects fresh dependencies. A lot of software in Manjaro repositories is themed and heavily customised, KDE Plasma included. Manjaro is also quite bloated out of the box, with snaps, custom kernels, a bunch of desktop software (potentially including some sponsored proprietary stuff, like FreeOffice), a bunch of custom theming (that looks pretty nice, actually), and a pretty awful GUI package manager (that unwillingly DDoS’ed the Arch User Repository several times and was even blocked by Arch for a short time).
EndeavourOS has the best community forum out of all Linux distros. People there are friendly, joke around and have fun all day, and are willing to help anyone. I think this is its greatest selling point. And also the fact it doesn’t bring too much beyond a nicely pre-configured and easy to install Arch Linux system, with almost no bloat, which depending on whether you want a heavily themed desktop or a more bare-bones experience, may or may not be a plus.