I think llamas are more aggressive and larger.
Llamas are bigger than Alpacas.
We had friends, who raised both and had more aggression problems with the Alpaca mothers.
They sold all of them, when they really got into raising wine grapes. Imagine a herd of Llamas and/or Alpacas getting into a vineyard and going down a line of grapes eating them.
The only thing worse is a big flock of wild turkeys feasting on your grapes.