Z9X and Z1000 Pro have the same amount of RAM (2GB). The difference is in internal storage capacity. This is used for storing downloaded apps and storing databases for Zidoo Home Theatre / Kodi etc. If you use the box primarily as a media player and have a library that isn't huge (1,000 items+) 16GB flash is fine. If you want to download and run lots of apps, particularly games which are often very large, or if you have a huge media library, then 32GB is better.
2A is marginal to power an external physical HDD but it will depend on the HDD. Physical drives have varying power requirements and flash drives / SSDs are much lower.
The BT remote does not require line of sight to operate, so the box can be hidden. It's got a few extra keys also, but not having to be in line of sight is the main advantage.
There are a few videos on Youtube of X20/X20 Pro interface. This is an early version of that found in Z1000 Pro. It gives an idea of what it's like. Basically, it provides a more direct way to access the Zidoo media apps and is nicer looking. The underlying apps are the same.