I've been to all four corners in China and, although I prefer Shanghai, I always recommend Beijing to a first-time visitor. The Great Wall and the Forbidden City are definitely two of the ten highlights in all of Asia (and definitely doable in two days). In comparison, some of the temples in Bangkok are nice but not nearly on the same scale. Also, late Sept/early October is a great time to visit Beijing. Bangkok may still be oppressively hot at that time and the traffic is bad. If you want to see more than the inside of a nice hotel, of which there are many in Bangkok, then I would recommend Beijing. Now, if you had more time and could squeeze in either Angkor Wat in Cambodia or Bagan in Burma, then I would recommend Bangkok as a base to see those two places.