The battery problems you heard about on the iLiad (5-6 hours) were fixed with a software patch. The wifi was turning on when it should not.
I get about 12 hours per charge with my first version iLiad (old, smaller battery). I think 15 hours with the new battery is a good estimate. If you want to be able to make notes on your PDFs you really need the iLiad.
If you want to see what the screens will look like, try cutting holes in cardboard to match the screen size. Put the hole over an A4 page. This is how much text you will be able to see. Try photocopying the page to reduce size until it fits the rectangle. Are you happy with the result? If so, you can buy that size screen.
The iLiad has the best support for PDF right now, with zoom, pan, and continuous pages, and the largest screen.