Wes is only 3 months older, so consider his greatest drawback a wash.
How did they produce this year?
Per 40 minutes, James outscored Johnson 23.8 to 18.9 and outrebounded him 13.6 to 9.7. James played 30.3 minutes per game, and Johnson played 35, so these very large gaps shrink but don't disappear when using per game stats. Shooting % is a virtual tie at .502 for WJ and .501 for DJ. WJ was better from the 3, at .415 to .385, but James got to the line a lot more (11.4 vs 8.8 per 40 min). Johnson has about 1 more assist per 40, and they are close to each other on steals, blocks, and turnovers. Defense can be argued about ad nauseum (which is OK with me), but the shortest version is that nbadraft.net gives them both 8/10.
My takeaway is that James is the better offensive player, scoring more without losing efficiency, and also a superior rebounder.
My other takeaway is that we got a steal for the #27 and #31 picks.