Both models are very helpful but still have differences…
The SAMR Model focuses on: Substitution, Augmentation, Modification, and Redefinition
The RAT Framework focuses on: Replacement, Amplification, and Transformation
The major difference between the two is the words used to define technology being used in the classroom. Redefinition and Transformation both allow for new tasks to be done through technology that were previously inconceivable. Another difference is that the SAMR model is divided further than the RAT, and in my opinion is overdone. The RAT Framework is simple but not over simplified like the SAMR Model.
Refer to the link on RAT Framework for further detail on it.