"If you wait out for Charge CD before using your 2nd charge you end up with a 10 second CD..."
If you do that, then after charging once immediately, then charging with the second charge at the 10 second mark, you do wait 10 seconds for your third charge, but then you have to wait 20 seconds after that for your second charge to be ready again.
So that will mean you can charge at fight time of:
0 seconds
10 seconds
20 seconds
40 seconds
and with Juggernaut you can charge at fight time of:
0 seconds
12 seconds
24 seconds
36 seconds
So for a 40 seconds fight, they are both "equal" in the maximum amount of charges you can pull off. The longer the fight goes on, the better juggernaut becomes.