I hadn't realized that QQ and WeChat are part of Tencent, but are part of different divisions. It would be interesting to know how collaborative or rivalrous the relation between them is.
QQ was a desktop focused app and WeChat was a ground up write of messaging for mobile. Competing against themselves is pretty brilliant IMO, it creates alternatives for the users but still allows their company to own the market.
Interesting YC blog about Tencent and WeChat. [1]
1. http://blog.ycombinator.com/lessons-from-wechat/