The 12Z is a little more advanced, the biggest difference is that it has a feature that allows you to trim in the plane, say for knife edge flight, while flying... then you can hit a button and it will automatically put the trimming you've done into a switch, and zero the trim settings. Next time you flip that switch, all you need is rudder to hold a perfect knife edge!
There are a few additional differences. You can go to Futaba's website for a detailed comparison...
http://www.futaba-rc.com/radios/feature-compare.html
I have the 12FG... love it, but if the Z was available, I probably would have bought it.