For the little bit of extra time it takes I would either use concrete, or set your posts at least 5 feet in the ground. I think concrete would be easier ;) After having my horses home and the amount of scratching, rolling (posts get kicked occasionally), etc. that goes on in the run in shed - they would eventually take it down if the posts weren't set in concrete lol.
Not to mention the occasionally hurricanes and strong winds we get would weaken it over time as well. During Sandy a tree fell on a part of the barn and only the edge of the roof was damaged. The posts and structure survived.