My brother built one very much like the second design. There were a couple of things. One, the smaller the greenhouse, the larger the temperature fluctuations. Bigger ones stay more stable. He turns on a space heater at night in the winter to keep his the temperature he wants. Some of his stuff is super tropical though, depending what you want to grow that may not be necessary. Also, he uses grow lights in the winter because he doesn't get enough hours of daylight during Seattle winters for some of his plants.
The giant shed attached is awesome and handy though. He keeps a large year-round worm farm inside his along with all his tools and stuff.