Which gerbil is bigger? Mongolian or Shaw's Jird?

I'm getting 2 gerbils, and I wanted to know which gerbil was bigger, because I want the biggest gerbil breed. So, could you help me out here? I need to know which one is smaller, and which one is larger. Please answer!!! (And don't say anything like, Gerbils stink, get a different animal!) Because I don't have a choice. And I'm LUCKY I'm getting 2. ANSWER!!!