Even though Casteroids are better at collecting wood, mammoths can carry so much more, and have higher weight reduction. They also move faster and have a larger attack range. When gathering wood on a mammoth, I personally have found that it’s stomp, or left trigger (Xbox) attack gathers more than its main attack
Although they don't have weight reduction Theris are extremely fast gatherers. I gather wood with my Theri and have a beaver following so that when my Theri gets encumbered I just throw it all on the beaver.