Robert S. said:
The Custom has a 14k+ wind and the JB is 16k+. The Custom has a ceramic magnet and the JB has an alnico 5 magnet. The output is fairly simular so it's a hard call to say which is hotter but I think the JB has a little more juice overall.
The Custom has a tighter and somewhat more bottom end and the top end is a little more agressive because of the ceramic magnet.
The Customs resonant frequency is slightly higher than the JBs which would indicate that the Custom might be slightly brighter but to my ears the Customs fuller bottom end and perceived (by me) lower placed mid hump actually makes the JB the brighter of the two pickups overall. I believe the JB has a thinner guage wire and the wind pattern is probably different which would also account for the difference in tone.
By scream I believe he means the JB cuts better in the guitars upper register. The JBs upper mid peak accounts for its exceptional cut during solos.