I would not get the PRS-700, because its screen is less clear than the others and you don't need its added features. The PRS-505, EZ Reader and Kindle 2 are all good choices, as is the Cybook if you can get a good return policy.
Amazon has a 30 day return policy, and you may be able to get a Kindle 2 by the end of the month (if it is announced tomorrow and you order a Kindle 1 today to get in line). A problem with the Kindle is that it has one font (6 sizes) and no bold option, so if the font and layout does not work for you all you can do is return it. It is also the most expensive, although used Kindle 1s may soon be cheaper.
The PRS-505 allows customization by changing the ebook itself. This is easy to do with Calibre, but it means that ebooks for different sources are not consistent. The Sony is the cheapest option.
The EZ Reader with the standard firmware is very good with FB2 ebooks and fairly good with MOBI ebooks. For FB2 ebooks you can change fonts (and always use the same font once you find one you like), but there are fewer font choices currently with MOBI. However, I suggest using the Open InkPot firmware which comes with two "reader" options (FBReader and Cool Reader 3) and allows you to install your own fonts. For examples of how the screen looks using the standard firmware and OI see BeBook font sizes and types
. For FBReader with a different font and Cool Reader 3 see New OpenInkpot snapshot released (now includes CoolReader)
. Once you have the font and page layout setup to your liking with FBReader (say) every ebook will look that way no matter where it comes from. An issue with OI is that it only supports DRM-free ebooks, but there are tools to strip the DRM from three of the most popular "Secure" ebook formats. This is generally thought to be illegal in the US, but it is hard to see who is hurt by doing this on ebooks you bought for your own personal use.
All Eink devices work very well in daylight and with "full spectrum" lighting. I was amazed how sharp the screen looked when I first viewed it under such a lamp. It isn't clear that lighting is your problem, but these lamps are worth looking into even for reading paperbacks. Some are expensive because of the "health" angle, so avoid those.