Punkfolkrocker: One great advantage of an app like Songbook (over both paper and PDF readers) is that they can re-size the text on the fly without any need to scroll right-left (impossible if you are playing too). I use a 10" tablet (rather than a 7") I got cheap second-hand and find I can set it to be seen clearly by my old eyes in any setting.

As for iPads being superior, I see no advantage if you are using a tablet just as a song storage/display device. Since I don't do anything with interfaced instruments, I would just be paying extra for useless (to me) extras.