Relay before or after power supply
Switching mains is better because:


It eliminates vampire losses by the power-supply itself which will use some power in standby. This saves a little bit on the energy bill.
It eliminates risk of fire from PSU overheating (although it may introduce it somewhere else).
It avoids heating the room.


The disadvantages include:


Thermal cycling of the PSU, PCB and components. This may lead to component or board failure.
Delay before you can print.