2 CPU's on the 12Z, 1 on the 12FG.
The 12Z has speaker volume control, and better pods.
The 12Z has 4 trim tabs, 12FG has 6 trim tabs.
You can compare both radios here: http://futabarc.com/radios/feature-compare.html

For me, the 12FG is the way to go.
Doug.