I would add that if you don't need the full functionality of the "One" (number of devices and rechargeable dock, mainly), that the 600 or 650 actually feel a little better in the hand (in my opinion) and add the four coloured buttons (which you can, of course, add to the touch screen on the "One" instead).
I also think that the 600/650's use of hard buttons at the side of the screen work a little better than the touchscreen.
All just my opinion, of course, but seeing as you can get a 600 for about £40, it's definitely worth it if you don't need quite so many devices.