I guess it seems like we ought to compare them, but Chris Johnson is six years older than James Young. No question Johnson is the better all around player right now- light years ahead on defense, better outside shooter, and probably more hustle to boot. Young is likely more dangerous with the ball in his hands, though, even today, at age 18.
I like Chris Johnson a lot, and he'll still improve, but odds are that by the time Young has a few years in the league under his belt he's going to be in a different stratosphere.