C3 has lots of new features like an improved image editor, array/dictionary editors etc, that make development process much easier.
Unfortunately, there are almost nothing new added in events/actions/behaviors.
Also, there are still quite a lot of bugs and unresolved issues.
So if you already own C2, it's probably not worth switching to C3 right now.
But if you don't have C2 license, getting C3 subscription seems like a better choice.