I am not sure if I understand, the Sony reader has the full page, A4 size, shown on a 6" screen, so it's effectively shrunk a bit more than two times, while the jetBook got the margins Brissed (almost 1.5" on each side), so the page fits almost 1:1? If yes, the no wonder the letters on the Sony look so thin, as antialiasing seems to kick in. You'd need to zoom in about twice on the Sony to give a better comparision.
Seperate issue is the yellowness of both screens. Maybe try putting a piece of white plain paper behind each ereader when taking a photo and use it to set white balance on the camera?