I've been to Beijing roughly a million times, but you may not shoot like I shoot. I end up overwhelmingly using a normal lens---for me that's 50 mm, although some people really prefer 35. I can see places where a wide would be useful, but less so a long lens; unless you want to shoot skylines with a "stacking" effect, or something, any line of sight that would justify a long lens is usually blocked by a building, a taxi, or somebody selling DVDs. :-)

Get out early in the morning, while the city is just waking up, and walk around the residential neighbourhoods before the crowds really form. In my experience that's the best photographic time in Beijing.