I had the 500, which I wasn't really happy with, so sold it and bought the 700. I find it much easier to use - not that I struggled with it, just didn't find it particulary ergonomic. The 700 is better built and more accurate. It's a fair bit heavier, but I think the design makes up for it.
I know I'm in the minority on this, but i found the 500 to be in no-mans-land: not great for cabinet work, too small for projects involving bigger pieces.
Having said that, the 500 is still a great tool and if there wasn't a 700, I definitely would have kept it. But for me, a DDF40 for cabinets and a DF700 for everything else is perfect.
The 700 looks huge, but when you actually pick one up it's surprisingly easy to handle.