I noticed about myself that more the number screens I have (in general mind you) the more distracted I usually get. An instance is/was my multimon setup. I was so awed by it initially that I would totally forget what I was supposed to do. Instead I'd waste time trying to "tweak" it, or something but sure as hell no actual work was getting done
In general I'm very anti touch screen (yes even with cellphones) and that was why I did not like the PRS ones. I believe I must change myself, but the convenience of operating blind far outweighs anything I can think of (for me ofcourse).
Thirdly, I'd like to mention that on my Kindle the only thing I hate with passion is the navpad. Give me the Joystick of the k2 any day of the week
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I'm not sure about the weight issue, but I noticed that on the iPad no matter how fancy or magical the device is, there is only a limited amount of time that I'm willing to actually hold it with one hand and operate it with another. Subconsciously I try to rest it on something (engineering nitpick: bending moment maximum on cantilever beams, which is what the Ipad acts as). I guess when you're using a device for an extended period every gram counts. But I still don't think it should make that much of a difference since it weighs just as much as a K2.
Oh and Happy Reading which ever device you choose!