JC is a brilliant game by itself with a lot of strategic and tactical options. RIII feels like a tweeked game for no other reason than keeping playing time short at the expense of gameplay. I was very disappointed with RIII.
I have three full games of RIII and a partial game of JC and my thoughts are the opposite of the responses so far. JC feels like a lighter game than RIII with more luck elements (Vulcan card is an annoying example). For serious play RIII, as an intro to block gaming JC may be better.