A few differences.
AT-Pro: 15khz
Large DD coil
Standard Mode(ACE series ding dong tones), Pro mode(Low tone, medium tone, high tones)
Able to notch out targets in all modes
Iron audio has distinctive low growls.
AT-Gold: 18khz
Smaller DD coil
All Metal Mode(deeper than AT-Pro, only low tone and medium tones, only able to change discrimination settings while iron audio is on.)
Two discrimination modes(one with everything notched in and the other has pulltabs and foil notched out. Low tone, medium tone, high tones.Discrimination works with iron audio on or off.)
Unable to notch out targets in any mode
Looks like you covered pretty well like I said the fellow I know that has both said he saw no depth difference in the two of his but maybe he has not tested out the all metal mode and I might ask is the all metal mode on the gold a no motion all metal or does the coil have to be moving ?
More ground balance features for hot ground conditions
Higher frequency makes it more sensitive to smaller non-ferrous targets and gold nuggets
Iron audio has higher pitched growling tone.
I think that's all the differences. Correct me if I missed anything.