Wow, it is a hard choice. Yes, for days is only enough for the tour in one place. To be Beijing or Hong Kong, it is a problem. It depends on which city do you refer to explor. They both have their own features. Beijing for the history site, imperial palace, great wall, hutong, while Hong Kong is for the disneyland, Ocean park, shopping, to see the metroplis scenery. You can also choose by where is much easier for your arrival or departure.