Cam Smith may be the better of the two but with the way the game is heading, Farah is more of an indication of what rakes will be in the future.
It seems that eventually the dummy half will be the lead playmaker (if its not that way already). Today the focus is on that halves but as attacking styles evolve we are going to see more hookers styled very similar to an NFL quarterback. Cam smith may create the platform for JT and Cronk, but Robbie will spend a lot more time controling play straight out of dummy half.
History will probably remember Cam as the better player but in 20ish years i think you'll see alot more playmakers styled like Farah than like Smith.