Nibs are generally considered superior in terms of the flavour they impart. They can however effect head retention because of their high fat content. For power it can leave a chalky residue and needs time to fall out of suspension. I am trying chocolate extract which ethanol based. You can also increase the amount of chocolate malt or use chocolate rye which is less bitter and intensely chocolatey.
You can add them to the primary or the boil. If you use extract you can add it at bottling. For nibs or powder I would add at the end of week two in the primary.
