Based on your need for PDFs, the iLiad Book Edition is probably your best choice. Its 8" screen really is better for PDFs than a 6" screen. The PRS-700 probably has the best PDF support of any 6" device, but its zoom mode is not persistent between pages and I think that will make your image based PDFs a chore. You might be able to pre-processes them into better PDFs for the 6" device though. Several other 6" devices will be coming out with mobile Adobe Digital Editions in the next few months, so there may be competitors for the PRS-700's 'best 6" for PDFs' crown.
The iLiad has limitations (e.g. its 12-15 hr battery life and no sleep mode), and there will be no further software from iRex for this device. Its built in reflowable ebook format is MOBI, and its MobiPocket Reader app is ok although with fewer features than provided on other MOBI-capable devices using the same base software. I always use FBReader on my iLiad for MOBI and for LIT (although I convert these to OEB first on a Desktop PC). Note that FBReader is DRM-free, and if your LITs have DRM this will need to be stripped (which may not be legal where you live) before they can be read on any EInk-based reader. The same is true of ePubs, which FBReader does not fully support - but Calibre can convert LIT and ePub to MOBI for FBReader or the native MOBI Reader.
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