I feel like it dims the colors a little. If you put it side by side with the 521GB, you'd see the colors are less vibrant on the anti-glare screen. They're like muted if that makes sense. It could also be personal preference, but I personally saw the difference and didn't like the colors on the 1TB.
Look into koreader. They host for you without an account. All you need is a username. You can read the same book and track your progress on different devices through koreader. If that's what you mean
Kobo is the way. Open system. Not locked like kindle. You can "jailbreak" it in 5 seconds. Linux based. They send you firmware updates even when it's "jailbroken". You can bring your own books and they won't fuck with your book covers like kindle does (I would use calibre to convert your books to kepub to get all the features on a book). No ads on the lock screen. I love my kobo Sage. I'm waiting for them to release a 10" one with high PPI so I can buy it. In all honesty, even the android ones are better than kindle. Basically, almost all non-kindles are good, except for a few random Chinese brands that have their own weird OS.
Fe₂O₃·xH₂O