I have a 650 (and a 350) and am very happy. I looked at the T-1s when they came out, and then the T-2s. I've installed PRS+ firmware on my 650/350, so have the ability to change fonts and many more customization options on it. The only thing I can see that the T-1/2s have that my 650 doesn't is wi-fi connectivity. And really it's not something I feel the need for--I'm perfectly happy transferring books to my readers through Calibre. If you're happy with your 650, I would suggest just hanging on to that. "If it ain't broke, don't fix it."
dordale