Don't buy the 600. It may be dropping in price, but the screen is pretty bad. I bought one and I was really disappointed after my 505. Almost returned it in fact, but even though I decided to keep it, I was never happy with it. I guess the same is true for the 900.
So I was really wowed when I played with a 650 in a Sony Store yesterday. The screen is simply amazing. Better even than the 505 and the refresh rate is noticeably quicker than all previous incarnations. It finally is better than real printed paper IMHO. Plus the new infrared touchscreen transforms the experience. The resistive screen on my 600 was unresponsive and often took more than one swipe to get the page to change. Setting bookmarks was a pain. That is all solved with the 650. Really responsive. Sony has also done some nice GUI cleanups and tweaks. The 650 is also much snappier than the 600. Basically, I bought one there and then
Only two gripes. Firstly, Sony doesn't even supply a slip case this time. Which at this price is just cheap. Plus it's shorter than the previous models so is the first not to fit the old Sony cases. You're going to have to buy a new one and they are not cheap.
Secondly, they've switched to micro usb and there's no standard charger socket. So another cable to carry around. There's plenty of space for mini usb and power sockets. Why why why Sony!
If you need the extra inches, then go for the 950. Personally, I think the 650 is the perfect portable size.