Wechat - like whats app with many extra.
Nowadays almost everything works via wechat or alipay. They have similar functions.
They are heavily controlled. Everything you post or do is recorded and ran through filters.
EVERYBODY in China has Wechat.
Weibo - more like twitter. Celebs spam their stuff there. Not so useful, many young people don't even use it anymore.
Not sure what your goal is, but since a few months, you cannot spam products on your Wechat moments anymore. I do not think selling products via wechat alone works that well anymore.