I'd recommend looking at the
matrix for ereaders. As others have mentioned, first decide what screen size you want and then decide what features you want and how much you're willing to pay. You mentioned running out of space for your library, which suggests to me that you might want a reader that can read flash cards. That disqualifies the Sony 300. Its internal memory is all you get.
This is strictly my opinion, but I would consider screen size, screen quality and clarity, available memory, including flash cards, and ability to organize the books on the device. Also, look at the ergonomics of how you would hold it compared to where the page turn buttons are located. I loved the screen of the 505, but hated turning pages on it. I've never seen the Jetbook lite, so I can't comment on its LCD screen, but I can say that eink screens, such as the 505 have, are very easy to read without eye strain.
If you're looking for a 5" screen, the Astak PocketPro is a very nice reader.