[stumped]

i don't worry about security as I am very careful not to load dodgy apps, and I always install malwarebytes AV. battery life is a potential issue though.
[ I went the route of spares in the cupboard with e-readers, once, and as I recall the advice on those was don't let the battery ever completely drain, give them a top up charge every 3 - 6 months ]
I am pretty sure that the older model has no native .mkv support but the newer one has, but for watching from my plex server, that's OK - stuff will transcode as needed.

I actually don't know what could happen if my newer device failed. I have the expensive square deal insurance cover via Amazon which promises repair or replace for another 2 years, but these things are not really repairable, and how on earth would they find a replacement ? So an insurance claim could -> money back instead, in which case having a cheap spare I bought earlier in the cupboard would be good.
As at now amazon warehouse have one at £130 and a couple on ebay are cheaper - but still being bid on.

As for specific performance - amazon video in HD is restricted to amazon tablets only ( all other android devices are capped at 480p streaming] and none of the current amazon tablet range have full HD screens, they are only 1280 x 800. The fire HDX is 1920 x 1200 - 1080p - it even says so when streaming, as confirmation. If I stream amazon video to my 10 inch Lenovo tablet it looks pretty awful in SD.
maybe a 720p amazon video stream on a cheap current model HD 8 would look OK but there is no practical way to test that. ditto with reading. The HD8 reviews say that it's browser fonts can look pixellated, so i'ts not going to be smooth like my Kindle voyage, for books.