I've read that your right rear tire will wear slightly faster than the left if you have an open diff. So I guess the right side would be the better place to put the new tire. I have not noticed significantly more tire wear on the right side on the 5 sets of tires that I've gone through.
If you want to get more life, flip the tires (remove from rim and flip over so the inside is now on the outside). My rears always wear on the inside much faster. That will give you about an extra 25% from my limited experience.