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It's only a ten quid difference between ads and no ads (they'll remove the ads from a version with ads for a tenner), fifty quid sounds like the difference between Kindle and Paperwhite (or the no ads version wasn't in stock and it was a third party seller).
I'd say a Kindle is still the best device even if you don't buy books from Amazon. Use Calibre to convert and email the books to your Kindle account and they will sit there to be downloaded and synchronised (to furthest page read) across multiple devices.
There is surprisingly little bloat on them compared to what you'd expect and if you do get the ads version they are as non-intrusive as they could be (I removed the ads on my main device as I like having the cover of what I'm reading as the lockscreen but I still have them on others)
It's more like £50 difference and there's no issue. My question is more along the lines of "what the actual fuck? It's supposed to be a book... and it has ads?"