If you're in the US, you are switching 120vAC, but I'd want a 250vAC rating for the safety margin. I personally would want a healthy current rating (5 to 10 amps) for longevity - the inrush current at switch-on is greater than the steady-state current.
DC voltage ratings are much lower, often 24 or 30 volts DC, but the power switch is handling only AC so that is not important.
Some switches are also rated for powering motors, which are a reactive (inductive and/or capacitive) load - that's a plus since the transformer is inductive and the power supply filter is capacitive. But there is no real need for a reactive rating in this application.
Incidentally, I'm pretty conservative about ratings. The risky thing is switching high DC voltages with an AC-rated switch, which sometimes happens with a "standby" switch in a guitar amp. There is a small arc every time in that case. But even then it takes a lot of switching to wear down the switch; most seem to last decades in spite of violating the specs. A replacement switch rated for 15 amps AC is sometimes used in that position to extend the switch life.
