What is dingux?
Dingux is an OS based on linux that you can install. It supports different software than the native OS. You don't overwrite the native one, you'll always have the option to start your Dingoo in one or the other, so you can have them both and use them for whatever you like. The differences between native and dingux have been mentioned thousands of times, do a quick search and you'll find it.
What is the difference between dingux and OpenDingoo?
Dingux is the original version of that OS, OpenDingux is an updated version. You should get the last one if you want to try Dingux/OpenDingux, it has less bugs and all the development is being made with that version in mind.