One caveat for Mac compatibility, if you think you might want to also be able to read your purchased books on the Mac without stripping and/or converting.
Both Amazon's Kindle for Mac/Kindle Previewer/KindleGen and B&N's Nook for Mac/NookStudy desktop readers apps require an Intel Mac that's at least 10.5. The Nook apps require 10.6. Sony's Reader Library app, horrible though it is, will still work with a 10.4 PPC Mac as well as the latest models.
So if you have an older model of Mac which isn't upgradeable, that may be something to keep in mind.
Aside from that, I have an older K2 and it works okay for PDFs and horribly slow for web-browsing (I don't think it's so much the speed of the connection as the clunkiness of the included browser.
Just about all the e-readers you've listed should be fairly equivalent in terms of what you want, although I've heard that Sonys may have a few slightly more useful PDF handling features (reflow, etc.).
Hope this helps, and welcome to MobileRead!
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