I think WhatsApp took a very good step and added a voice chat feature in it. WeChat was getting popular in very small span, due to its voice messaging service. But I personally think that it was not that user friendly application comparatively WhatsApp. WhatsApp is very simple to understand and use. Using two messaging service is not worth. Even WhatsApp is giving better voice quality than WeChat.
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There are lot of IM apps, the basic purpose which all of them does is to replace the good old SMS. Whatsapp is something which is best at it. I have been a Whatsapp user for almost two yrs but Wechat installed it gives stiff competition .
One advantage that Whatsapp has over Wechat is that is has large user group worldwide, so most of your friends will be using Whatsapp, but that's not the case of Wechat as the app is still a month and half old. Wechat still has some neat tricks in the book which Whatsapp doesn't. And ha Whatsapp is not free like WeChat.
One of my favorites being the voice chat. It's more like a walkie talkie style conversation and it's pretty fun. More than text messaging it actually replaces your regular phone calls.
I haven't tested Wechat on WiFi yet but performance on 2G have been very impressive and messages get sent pretty quick. The only shortcoming is that it doesn't have that double tick system that Whatsapp has or the last time the contact is online.
With all this being said, there's are some slight issue which will get fixed by the next update
However, WeChat has been advertised in India very aggressively and should be able to amass substantial number of users in very short span.