I would add the Acer A500 to the group. mainly because of the full size usb port. there is no need for the docking station/keyboard to get the usb port. it will be upgraded to ics in early 2012.
7 inch vs 10 inch is a matter of taste. 10 inch better for viewing video, 7 inch easier to hold and thumb type on
the kindle will do fine for video and ebooks but I'm unsure about doing any serious work, but Amazon app store does have docs to go and quick office. If you want a full gps then the kindle is not the one.
Written on my Acer A500