In my area the difference between round and square comes down to the treatment applied to the wood. The round ones can de sourced from our farm co op type place and have a high grade treatment. The square stuff is sourced from the lumber yard and doesn't share the same treatment quality.
Either way I'm a steel post with the L brackets guy myself. Especially if it's only a few hundred feet at my entry. I dislike fence work so much I don't ever want to do it twice....