ALL electric space heaters are exactly the same efficiency- 100% of power turns into heat. The differences are in noise, size, longevity, etc- and some may have better controls so it isn't just full on / full off, and some may have more surface area to reduce hot spots in case they get touched. All will be limited by max power draw for a normal 15 amp circuit unless you get a fancy 240v unit and run a new circuit, but that would be overkill for 200sf. You could easily but probably not cheaply do this with a solar powered space heater,
I'd recommend just getting a space heater you like, that isn't too flimsy, but I'd also recommend something like a black blanked at the other end of the sunbeam to absorb and release the sunlight more effectively.
You CAN also make a solar heater if you can put things outside your window, the "black painted cans" is a tried and true option. This may take some fiddling, but you can build something nearly free. I've seen these a lot, but here is a random google search result I found to match what I remembered. I cannot vouch for this actual blog, it was just the first result.
http://www.reuk.co.uk/wordpress/solar/make-a-simple-solar-air-heater/Additional options here potentially worth looking into:
https://homestead-and-survival.com/14-ways-to-heat-your-home-for-free-with-passive-heating/ -- you will still want a cheap space heater for nighttime use, but they do use a lot of power for what you get. (Remember 100% electric efficiency is rather poor compared to the 300% you can get with a heat pump, and isn't great for the climate or your wallet)
