"Acid stop" can either mean acetic acid or citric acid.
From my notes:
Acetic acid: economical, effective, not risky for the film, dangerous for persons, polluting, stinking, reusable, well preserved; can be used more times when diluted at 2%. For one-shot better using 1% or 1.5% dilution.
Citric acid: more expensive, not dangerous, odourless, must be used on the same day of dilution, and some people report the possibility of problems (my notes not very specific).
I chose acetic acid. I still have to perform my first C-41 bath, my "chemistry" is still sealed. Hope I'll do it in the next few weeks.