The biggest issue is the library books - if your mum already uses the library, she's almost certain to want to use the ereader with it, and the Kindle just simply will not work, and there's no indication that's going to change any time soon.
Personally, I vote for the Sony T1, but that's because it satisfied everything on my quite picky list. OTOH, if you want to get a cheaper reader, the Kobo's currently being sold at around £67 (Waterstones, Asda, Currys), and it does work with UK libraries.
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