The jc120 is Solid state. The twin is tube. Most would say that the jc120 is "sterile" and they're right. The twin will color your sound, but in a way that is good for basically any style. And the twin sounds infinitely better with pedals.
Except for the price, I can think of no reason to pick the jc120 over the Twin. Of course, in the end it all comes down to your personal preference, so play them both and pick whichever one you like better. But if you won't have an opportunity to play them, I would definitely say get the twin.