Just the three I own:
Sony 505 which was my first purchase. It has a BookArmor case so it is my travelling reader. It will go on vacation with me. Like it very much, although the page turn buttons bother my right thumb somewhat.
Sony 700 which was my second purchase. Bought it off Craigslist on a whim and used it twice. Loved the light and larger fonts but could not master the page turn sweep and was forced to use the page turn button. It lives in a drawer.
Kindle 2 which was my third purchase and my favorite device. Love the larger screen fonts and the page turn button.