Can someone tell me why Rags are better than SCIDS? I've heard that SCIDS are leaky. Pointing me where to find this information would be great too.

Try this reference:

Meyerrose TE et al. Immune-deficient mouse models for analysis of human stem cells. Biotechniques 35:6, 1262-72 (2003)

"The current review discusses the merits and drawbacks of each immune-deficient mouse xenograft system as it stands to date and reviews how each immune-deficient mouse model has been used to further our knowledge of human hematopoietic stem cell biology."