The only difference is that RD20 says that is a greenhouse disinfectant
for orchids; Physan 20 gives instructions for use as a disinfectant for
swimming pools, dairies, hospitals and restaurants. BTW, the warning
on the label of RD20 is rather scary.
In his book on North American CP, Schell (1976) recommends benomyl for
fungus problems and none of my other books makes a recommendation for
the problem. I'd be a little wary about using benomyl now since DuPont
has removed its brand of benomyl (Benlate) from the market after losing
a number of lawsuits brought by commercial orchid growers a couple of
years ago.
Sorry I couldn't be of more assistance.
John