While I do respect other people's experiences, I would like to say that cockerels can go through a personality transformation at about a year old when they become roosters. Cockerels that were sweet and docile can become aggressive, and cockerels that were aggressive and bullyish can calm down to be very nice roosters. Mine, that was obnoxious as a teenager, will now put his head on my shoulder when I pick him up and he really likes chest scratches. My alpha rooster (the one in the background in my avatar) was always sweet and let me pick him up when he was a cockerel. At about a year old, he suddenly decided that he didn't want to be picked up anymore and adopts a "don't touch me, don't touch me" retreat.
As far as keeping an aggressive rooster, I say no. My point was that it's too early to make that call at six months because the 'nice' cockerels you have now might become terrors at a year old and the one you like might be a fine rooster if given a chance to grow up. I'd hate for you to make a call now, get rid of the one you really like and keep a 'calm' one only to have him do a Jeckel and Hyde at a year old.