As solitarywolf has stated the screen on the 600 is nowhere as good as the 650, it looks black and grey rather than black and white. But if that's all you can afford it's better than nothing.
I have both the 600 and the 650, not that I use the 600 any more, but during the time I had only the 600 I was more than happy with it.
The touch screen is not as responsive as the 650, but still perfectly usable.
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