welcome to mobileread!
I have only experience of the Pocketbook 360, and it scores full marks on hackability and portability (it fits into a pocket of a jacket). The newer version, 360 plus, has wireless support, and you can find a review
here. As for hackability, there are many things you can do, from
installing ready made themes, to making your own, to installing additional reading softwares, as e.g. the FBreader 180 that allows you to turn pages by flicking your wrist. The buttons can be reconfigured to do many things. Screen is 5'', and is not Pearl, but I still find it good enough. I do use it regularly to read technical pdf files when I am on the go, but some people find anything below 6'' impossibly small for pdf and djvu.
For the other readers you are researching (and for more impressions on the 360), I am sure other people will chime in soon.