Core vs Reactor?

[eluser]k2zs[/eluser]
Hello all, I've been away from CI for a few months and see that 2.0 has been released. I am wondering which version I should go with, core or reactor?

I see there is a "core" folder in the application directory, is "My_Controller" still considered a library or is it now to be put in "core"?

Also... Is modular extensions supported in either of the 2 new branches?

Thanks,
Scott