The contrasts between the two burial postures have to do with what the positions represent about the afterlife of the dead. The Native Americans are being buried together in sitting positions, as if they're still alive and spending time together. Christian burial traditions, on the other hand, leave people in their solo graves in a horizontal resting position.
The speaker in the poem focuses on the Native American traditions. He says that though life is over, the activities and meaning of that person goes on. The dead are still sitting at the tables with their friends, partaking in their routines. They don't just fade away. Death isn't only restful but rather the next step in a journey.
From the speaker's understanding, the Native Americans don't see death as going to sleep forever. Instead, the speaker believes that they see death as active and engaging—like life. The spirits of the dead live on and continue to roam the woods and engage in the same kinds of activities they might have in life.
Ultimately, the contrast between the imagined afterlife—the peaceful Christian afterlife or the active Native afterlife—is the largest contrast in the poem. The positions the people are buried in are only emblematic of what happens to their spirits once they're gone.