I think a bit depends on the setup that needs to be guarded and how, this is an opinion. If you have multiple coops or need a mulitiple purpose dog, the GSD is the best in the world at guarding and intermingling with ranging fowl. However, if a stationary or fixed setup is what you want, the GreatP is a lot more laid back, and does not seem to have to work too hard to be satisfied. My experience is that the GPs work best in an enclosure of some sort, behind the fence in other words.
Most dogs have to be trained not to eat chicks, ducks etc. If they mess up, it does not mean that it is a permanent trait or issue. Sometimes it is mistaken identity, my owners forgot to feed me or gave me a bag of carrots as a meal and this looks a whole lot like fast food coming my way.
My wife Ruth has posts on our GSD, Rex somewhere.