Well today my wife and I went for a test drive in both, one after the other, to help us decide. It is going to be difficult and it may come down to the deal in the end. Impressions were:-
Yaris - more modern, more stylish, more techy kit like bluetooth, DAB radio, parking aids.(essential) Quieter around town, heavier feel to steering, higher quality feel inside and better dash. I think will be more economical on my wife's majority journey's of under 5 miles.
Jazz - lovely roomy, airy feel to interior , better visibility. Colour changing dials both useful and fun but I would possibly spend too much time looking at them! Plastics seem cheaper. Very smooth and easy to drive although so was Yaris. Obviously more versatile storage options.
Both were great to be honest.
My gut feeling is that she is going to prefer the Jazz for the space and me the Yaris for the looks and gadgets! So not much progress although initial dealings look like the same level Yaris will be cheaper.
Dave