1 - What are the BIGGEST differences between C2 Runtime to C3 Runtime?
The C3 runtime is a complete rewrite with a whole new architecture, much better performance, and new features (e.g. BBcode in Text objects).
2 - Is C3 Runtime much more POWERFUL compare to C2 Runtime or there are only minor changes?
The C3 runtime is much more powerful. It already has major new features. In future major new features will also only be written for the C3 runtime, and its new architecture makes that far easier and allows for much more significant changes.
3 - How can I Test/Feel some differences right now on my game using the current C3 Runtime stable release? (R111)
Performance tests in intensive games should significantly improve, or you can already test the new features.
4 - Should I keep working with C2 Runtime for now on my official game project? or not? and why?
The C3 runtime is still experimental so you should probably stick to the C2 runtime until we announce that it is out of beta and officially recommend that projects start switching to it. It's still useful to try testing your project with it though to see if it works and report any bugs that come up. You can then switch back to the C2 runtime providing you didn't add any features only available in the C3 runtime.
5 - Will C3 Runtime be complete and default soon? and is it already safe to use it on official project?
The C3 runtime is pretty much already code-complete. The only remaining major step is to finish the C3 debugger (again being rewritten), and keep fixing bugs which will probably keep coming up for a while since it's still relatively new. But yes, we should be ready to launch it soon, in the next few months hopefully.