You can compare the 5 series boxes here:
Code:
https://buzztv.com/5-series-comparison-chart/
All 5 series run Android 11 (vs 9 on the 4000 series).
The E5, Classic, X5 and U5 all have the same processor. The E5 has 2GB of RAM and the Classic, X5 and U5 have 4GB of RAM.
They vary in the WiFi, and the amount of onboard storage. The U5 has the option to install a 2.5" SATA drive (I recommend and SSD) directly under the unit (small compartment), which is handy if you do a lot of recording/downloads.
And the Classic has a different remote (BT-250) than the X5 and U5 which come with the BT400.
The B5 is the entry level, and if all you do is IPTV, it works quite well. No bells and whistles, and has an IR only remote (not BT).
The E5 is a step up. Same CPU as the big boys, but less RAM and storage, yet it also has an IR only remote.
The E5 and B5 only have a 10/100 Ethernet port, while the others have GbE. Not really an issue.
The HD5 is a little bit in it's own little category and great for portability with it's integrated HDMI cable..
That is pretty much the summary 
EDIT: Oh, and the HD5, E5, Classic, and the X5 A(X-C variant) use a USB C connector and adapter to power them. The B5 and X5 (non AX-C variant) use a 5V (standard barrel tip) power supply, and the U5 uses a 12V power supply (due to the integrated SATA drive option).