If you're a post frame contractor building pole barns and other post frame construction, your needs are most likely different than somone building a single pole barn or post frame building. Which bracket you choose really comes down to how many posts you're putting in the ground annually.
For the home owner building a single structure with a limited number of posts, the nail on bracket is a good fit for this scenario. Why? The time factor isn't as much of an issue.
We've all heard the phrase "time is money", this is where the contactor and the (press on) bracket is better suited. Obviously when you're building post frame structures as a livelyhood, you want to streamline the process as much as possible to make it as efficient as can be. With the (press on) bracket the post frame building contractor can press bracket on in bulk at the shop and pull from this inventory as a job requires. Brackets already pressed onto a post and ready to set in the hole is a hugh time saver; especially when you consider once setting the pole, you are ready to pour concrete and move the project along.