Deciding between NVIDIA Shield & Mi Box

I currently have a WD Live Box, and am thinking of moving over to either an NVIDIA Shield or Mi Box. I understand the NVIDIA Shield is considered the better box, though I’m just looking for what suits my current needs.

I will use it mostly for movies stored on two 4 TB hard drives via KODI, as well as Android ‘stuff’. ‘Stuff’ meaning, I am new to all this and do not fully understand much about Android yet other than there is streaming content, games, ect. (yes, still learning)

On my HDs I have (read this from my WD Live screen) mostly two types of files. ISO MJPEG and MP4 H264.

Also I would want to be able to expand if needed. The NVIDIA Shield seems to be easy in this regard but the Mi Box is questionable. As to the Mi Box if I were to connect a USB adapter Hub such as the ones that will add three USB ports and an ethernet port. Could I then use two usb ports of the adapter to attach HDs AND use the third port to expand the internal memory of the Mi Box ? Or will it just see either internal memory or the external HDs, and not both simultaneously ?

Lastly do these boxes actually shut off completely or do they just power down/sleep ? And will they also completely shut down the attached HDs ? I thought I read (?) where they might leave the attached HDs ‘on’ even after powering off the box.

Which box would be best to fit my current needs ?