I'm not sure about IQ, for example Modric looks fairly stupid to me listening to him but looks smart on the filed. You call that fooballing IQ but maybe is just talent and vision and IQ is IQ.
Alen is arrogant, has temper, while Kovacic is calm, religious guy...but for example many say he lacks courage, maybe he just doesn't see write moment to do write thing what can also be called IQ?.
So at the end Kovacic is more likeable guy than Alen but I'm not sure that he is smarter nor more talented (actualy I rate Alen talented).
When we talk about talent, people mostly think about technique and dribbling skills (usually).
People will usually say: Halilovic or Adama are sooo talented.
Not too many people will say: he is so talented because he has awesome tackles...
Halilovic has awesome technical talent.
But he is very poor in tactical elements and his footballing IQ is more or less horrible, especially for Barca's level.
But yes, he has balls/courage.
Kovacic isn't as good dribbler as Halilovic, that's true.
But he is tactically much better player both in attack and in defense and in workrate/buildup/movement for the team.
Also, Kovacic has much better vision, playmaking ability, ability to predict 5-6 moves in the future etc.
While Halil usually takes the ball on the center, dribbles, runs past 3 defenders, shoots from the edge of the box and either scores or not.
Kovacic wasn't too brave in the past (brave like Messi, Neymar etc).
But he is improving. Plus again, young Iniesta (from how I remember him) was a much weaker player aged 20-21 than the current Kovacic, and Iniesta was equally or even more scared/without the balls/courage. And look at him today.
So, about talent, there are 10 or even 100 factors about which we could debate.
So, Halilovic surely has more potential in terms of dribbling and scoring.
But in 100s of other footballing factors (physical skills, tactical skills, mental skills), he is behind Kovacic, imo.