If you haven't already invested project assets into a large project, there's little reason to go with C2.
The only reason you'd go with C2 is because it uses a critical plugin that hasn't been ported to C3 yet, and because changing versions/software in the middle of a project is foolish and incredibly risky.
As already said, they're pretty similar - so much so that the majority of learning material for C2 (of which there is a lot!) applies equally to C3.
In fact, C3 has some nice new features which streamline workflow, and because it's browser based you have ultimate portability and the ability to work anywhere with it - you just log in - this is something that I really like, personally.
There are a few things I wish were in C3 already (saving a project in a file structure for instance), but they will get there eventually and they're not a deal breaker for me. I don't think they are for most people, if they're really honest.
The majority of people who complain about C3 vs. C2 are really just trying to avoid admitting that they don't want to pay a subscription rather than a one-time fee. Which is a shame for them, because that's the way the whole software world is going. But people are entitled to their opinion, regardless of what I think of it.