You have to remember that most people aren't power users. A lot of people find if difficult to even install Windows. Vanilla Arch isn't for everybody.

Flatpak is the best one imo. Never used appimages, and snap is pure trash (close source, slow, made by canonical). Overall, native packages are imo the way to go, but flatpak is also fairly good.

As someone who tried both, I think Endevour is better. 1.It's more bleeding edge. 2. It's as close to vanilla Arch as you can get with a gui installer. 3. The dev team seems to be more compitent then the Manjaro team (i.e: shit doesn't break because someone pushed a WIP package). 4. Better community support (I mean, it's literally just Arch with a fancy installer).

They're both fairly easy to install. And it's fairly easy to switch between the two.