Shaw's Jird or Mongolian gerbil?

I am getting 2 gerbils in 3 weeks. I was wondering what kind of gerbil to get. I think Shaw's Jird is bigger than Mongolian but I'm not sure. I thought that Mongolians were longer but I'm not sure of that either! So I guess I need a little help. Oh, and I want the biggest gerbil. :)