Common tdonline, on Aura One you can easily remove header/footer and make a lot of other things you can't on Kindle Oasis since it is not hackable at all (for now at least). And moreover you can't say 8" is the same as 7", it is nonsense.
Bigger screen is better in a lot of cases.
But for me personally I read much more on Oasis than on Aura One. Just because it sits in hands better, lighter and has physical buttons. Aura One has a definitely better screen and backlight but still I read much more on Oasis. Aura One is the device on which I read bigger pdfs and technical literature (rarely), but for fiction I find my Oasis suits me better.
So they are simply different devices. I need both

and I am not going replace them by one new Oasis. I think if you have an old Oasis you don't really have a good reason to buy a new one. Only reason if you really ebook-addicted and want a new toy.