I'm treating my clown, two damsels and flame angel for ich right now. I chose to use hypo and it's working well. I didn't take the salinity down fast enough (I lowered from a SG of 1.026 to 1.009 over ~72h) so the parasite had a chance to multiply more than it should have, and I initially feared that the hypo wasn't working. Now after more than a week the fish are active, spot-free, breathing normally and eating well. I will keep the SG at 1.009 for another five or six weeks, then raise it slowly over a week and hopefully these fish will be ich-free for the rest of their lives.
I don't know much about using copper, and haven't heard of using formalin for ich. I chose hypo because it's hard to mess up - patience and a refractometer are the only tools you need.