You'd be surprised, there are a lot of people who think the 505 is a much better model than the 300. As far as screen quality and processor they are the same. The 300 has twice the memory, but the 505 has the ability to increase it's memory up to 8GB and SD is cheap.
The 505 has a 6" screen and the 300 has a 5" screen. But I think that is a draw, for me I like the portability of the 5" but like the 6" since I read a lot of PDF on the screen. If you and your sister read mostly novels then there is no screen advantage.
One thing is I like the button layout of the 505 the keys are laid out that you can change the page with either the left or right hand so holding the book in one hand is possible. I've not held a 300 but it is smaller and it might just be possible to change the pages with either hand.
The last point is the 505 has one of the best accessories available to it and that is the light wedge reading light. Personally I love/hate that wedge, it's heavy and scratch easy if not careful, but it by far provides the best lighting source out there with out a single glare.
I do think the 300 is a nice device, but if I had a choice and SONY still sold the 505 for a little more than the 300, I think I would still pick the 505 over the 300. But then again if I was shopping for a model I'd probably pick the 600 over the 505.
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