I've never used tr-20, but i've been messing around with tr-55 for a few months.
I think a lot of people use them together. Apparently Win tr-55 can output data that can be input into tr-20.
Tr-55 has limitations. It seems good for basic hydrology modelling of multiple drainage basins, but does not do any hydraulics and has limited storage modelling capabilities. Tr-55 is nice for calculating time-of-concentration, composite CN values, and generating scs runoff hydrographs and peak flow rates.
Hopefully other people will elaborate on these programs as I am curious to know more as well.