It all depends on what you are trying to accomplish with the accessories. Hole saws are great for really large dia. holes, as well as making clean holes for finished material (door nob installations, plumbing fixtures, etc.) but they are slower than spade bits, self feed bits, or ship auger bits. You can also use forstner bits for clean holes, but they do not come in as large of dia. sizes as hole saws. For a chicken coup however, I would guess that you do not care if the hole is "clean" or not, so I would agree that spade bits are a great option. As far as the DeWalt bits being nail "proof", that is not quite accurate. They have a better designed shoulder on the bit, so that they are able to cut through nails, but that will still dull them. The big benifit is that if where to hit a nail with any other competitive spade bits, it would ruin them, making them not cut at all anymore, as apposed to just not cutting as well. I have a bunch of the DeWalt spade bits and love them. Plus, they are impact rated, so you can used them in your impact driver as well as your drill driver!
Is it easy to sharp the dewalt spade bit after hitting a nail? My irwin spadebits have like small "ears" that always get ruined after hitting a nail and sharp them up is not a option. You need a different file for different size on the spade bits due to this "ears" or maybee I should call it cutting teeth or something like that. Anyone who have tried those TUNGSTEEN CARBIDE HOLE SAWS? They are quit expensive so would be nice if anyone knows if they are good. I will proberly try them out. Working as a electrician and are making a lot of holes for spotlights, outlets and light switches.