Better item fights are useless. It always comes down to what is the best item for YOUR setup. Generally speaking CoA > Shako, but I'v had many a build where I didn't need/already had what CoA gave, and Shako was FAR superior. I suggest you design a build with each one, then compare them. That's the only way to see which is better for you.
If your setup needs more DR and an extra socket for more Ias or whatever, then CoA wins. Otherwise, shako > CoA in most cases. For a few matchups, the extra 300 defense and/or 30FHR of CoA can give you a slight advantage.
IMO, depend on ur setup. For most smiter or hammardin, shako is the best helm for them. (+2skill, extra life/mana) CoA, on the other hand, require huge amount of STR if u dont socket with ed/-req jewel. And most likely, CoA will be used to melee characters such as zealot. the +resist, fhr, and +1skill wont hurt. Is really depend what u gonna make, however.