An OR power supply circuit
I believe you could do it with a SPST (or SPDT) relay IF you can tolerate having 3.3V appear momentarily at the 5V output.  If not, you'll have to put in a diode on the 5V supply but remember that it will drop 0.7V.
Edit: on closer inspection you'll need D1 to prevent the 5V supply from powering up RLY1's coil.  This also prevents the 5V supply from shorting into the 3.3V supply, but you still have the diode voltage drop.



simulate this circuit &ndash; Schematic created using CircuitLab