Power steering fluid vs. ATF

[lwjanzen at hydro.mb.ca (Janzen, Lawrence)]

Well after ten years, my '87 Scirocco is back on the road!!   Oh... what
a feeling!  :)  My wife said I was like a little kid on Christmas
morning!! Anyway, I have changed all the fluids except for the power
steering fluid.  The RB manual recommends Dexron or Dexron II ATF fluid.
My question is...what is the difference between ATF and power steering
fluid?  I am planning on putting in new Dexron III ATF unless someone
recommends something better.

Thanks,
Larry 
'87 Scirocco 16V