I was only really concerned with how they did at United/Madrid and Internationally;
In his career at United, Hernandez has scored in all competitions 50 goals in 117 games. A goal every 2.34 games.
Higuain at Real has scored 122 in 266 games, a goal every 2.18 games. (and 46 assists)
Internationally Higuain has 20 goals in 32 (and 9 assists), meaning a goal every 1.6 games.
Hernandez has 35 in 53 meaning a goal every 1.51 games.
Higuain's club form is superior and over a great period of games which makes it more impressive, he also gets more assists which ties in with him being the more rounded, technically superior player, based on current achievement.
It is interesting to see the percentages of games they started for goal, or the goal to minute radio. I'll bet that Hernandez has better stats.