Zidoo is apparently heavily geared towards playback using its internal storage. Is there any benefit to using internal storage vs streaming from another PC on the network (via DLNA/Plex) ? Seems to me this may be just another step - copying stuff to the box.The Netflix and Amazon apps allow you to download movies/shows to your local harddrive. This allows you to watch the content without having to stream from the internet on the fly. Other streaming services may have similar options. You can probably download content from services such as HBO/Showtime/etc. And, like you indicated, you can always copy your own content files to the media player drive if you want.
I kind of leaned towards Zidoo. Looking at things now, the Zidoo and Dune top models also include analog outputs. If you are just connecting to a processor using HDMI, you don't need these analog outputs. The Zappitti Pro 4K HDR top level doesn't have analog outputs, so you're not wasting circuits/money. Zappiti does have an "Audiocom" version of the top level which is even a step above, but I doni't see it available in the US.
As far as which is the best, it's hard to say. All 3 are Android OS based. All 3 have bugs and problems reported by users. When I was researching, it seemed that Zidoo was more responsive to bugs and has a larger community forum for posts/threads. I'm sure all 3 would be a good choice.