Doesn't a touchscreen overlay reduce (somewhat) the contrast and clarity for the e-ink? Every device I have with a touchscreen, the diffused matte surface reduces the clarity of the LCD. In low-light, this could be significant.
Obviously, the front-light will help in really low-light situations, but I wonder if the trade off the rest of the time is worth it.
Still, if I didn't already have a 505, I'd probably consider the 700.