SAFE400 Nuclear reactor.
According to some sources; 656 kg (512 kg mass, 72 kg per heat exchanger. If by "heat exchanger", they mean Brayton cycle electrical power generator, that's 2 * 72 (Two brayton cycle electrical generators).)
(Bottom of the second article down)
[link]Other sources, 1,200 kg.
(May include mass of stationary structures that would not be scaled up or included on a spacebourne design.)
[link]Power output: 100 kW.
So, 12kg/kW - 5.12 kg/kW.
Or, the Los Alamos reactor:
[link]2.03 kg/kW !
I wrote an email to Nyrath, the author, asking if he had more information on the Los Alamos MW reactor, and if that figure included a closed-loop cooling system.
Solar Panels:
[link]"20 to 100 kg/kW. The current state of the art is about 45 kg/kW."
At Earth distance. You'll need 2.86x that mass for a Mars mission, for roughly ~35% sunlight. (Unless you plan to never need to use your engines at aphelion.)
57-286 kg/kW. State of art: 128.6 kg/kW
For VX-200, 200 kW: 25,720 kg. 25 tons of solar panel.
Versus
2,400 kg to 406 kg of nuclear reactor.
More info:
[link]