Django vs webapp2 on App Engine
Choosing between Django and webapp2 really depends on what you're using it for.  In your question you haven't given any of the parameters for your decision making, so it's impossible to tell which is "better".  Describing them both as "web frameworks" shows you haven't done much research into what they are.
Webapp2 is essentially a request handler.  It directs HTTP requests to handlers that you write.  It's also very small.
Django has a request handler.  It also has a template engine.  It also has a forms processor.  It also has an ORM, which you may choose to use, or not.  Note that you can use the ORM on CloudSQL, but you'll need to use Django-nonrel if you want to use the ORM on the HRD.  It also has a library of plugins that you can use, but they'll only work if you're using the Django ORM.  It also has bunch of 3rd party libraries, which will also require the Django ORM.
If you have portability in mind the Django ORM would help a lot.
You'll have to make your decision comparing what you actually need.
I would like to know your opinion of which of these two web frameworks (Django & webapp2) is better for using on App Engine Platform, and why?
Please don't say that both are completely different, because Django is much more complete. Both are the "web frameworks" you can use in App Engine.