Relay or Transistor for DC Resistive Load?
I would use a MOSFET and the simplest arrangement is an N channel load switcher: -



Ignoring the requirement for switching it on and off at high speed (i.e. kHz upwards), when operating at 200V, certain precautions may be advisable. The first relates to the common earth/0V/ground point. You may want isolation between control signal and the load ground so you may want to go to something like this: -



You'll need to generate a small 12V isolated supply (note B2) and this can be done with a multitude of isolating dc/dc converters such as from Traco and others.

You also need to pick a MOSFET that is rated probably in excess of 300V and there are plenty to choose from here. Given that there may be some back-emf due to load cabling I would still use a reverse protection diode across the load to snub-out any high voltage spikes from damaging the MOSFET.

My 2nd advise is be very careful with the 200V when building something and testing it - make sure it is current limited (fused to prevent a fire) and, if possible find a dc residual current detector to switch off the supply should you get connected to the 200V like this one