Alright, I did a little bit of research at MG Chemicals, who produces the three common copper etching solutions.
Website here:
**broken link removed**
Some simple facts about the etching solutions:
1. Ferric Chloride
A. Stated as providing a more consistent etch than Ammonium Persulfate
B. Stated as etching twice as fast than Ammonium Persulfate
C. Roughly 10-20 minute etching time.
D. Will not remove most etch resist inks.
2. Ammonium Persulfate
A. Translucent liquid allows the user to see the progress of the etching process.
B. Roughly 30 minute etching time.
C. No flash point stated on MSDS, so it is not directly flammable.
D. May remove some etch resist inks. Must use a compatible etch resist pen.
3. Sodium Persulphate
A. Flash point at 180 deg F. FLAMMABLE!
B. The solution is shock sensitive
C. Solution decomposes when heated.
D. Cleaner solution, allows increased visibility during etching.
E. Will not remove most etch resist ink.
F. Must be stored in a ventilated container due to gasseous nature.
So it looks like Ferric Chloride is the most common, most stable, Most docile and consistent etchant solution.