Usally charge time does not matter. I typically connect my Sony to add/remove books so I am always charing. I think you will feel the effect of having to charge the Kindle more because it does not require a PC.
However there have been a few times where I have been VERY glad the PRS-505 has a long battery life. Once on a week long business trip the other on a vacation. Since I did not have easy access to a PC on either trip I when a week using my device w/o a charge. Both times I took the sony down to 1 bar. It was nice not having to stress about having to find a place to charge my device.
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