The awesome photo thread on pack animals has me wondering about disadvantages and advantages of certain animals.

When I retire in a couple of years we plan on getting a couple of mules.
The photo thread has horses, mules, and llamas. I even remember seeing goats in an article a while back.

So for you folks in the know; why do you prefer a type of pack animal?
What are the disadvantages/advantages.... from my experience with mules I know what to expect there.

Thanks in advance!