In my opinion it is also not a thing python vs c++. When I start a project I have a look at the available frameworks and librarys which are available to handle the features I need. I think that should make the decision. It's very hard to write most things again from scratch.

Personally I would not start to work on such a framework. The linked discussion on gnome mailing list about telpathy says all about this approach.

But hey, this is open source. Everyone who has a need is free to implement things that make his life easier. I wish you all success with Transponder!