Differences Between Transformers and Interceptors
An interceptor can be used to intercept the request/response at various points, as discussed above. But transformers can be applied just before sending a request or after receiving a response. Another important consideration is that, using transformers, you just get access to data and headers of request/response while interceptors have access to the config object.
From AngularJS - Novice To Ninja by Sandeep Panda.
Also, I think interceptors are global while transformers can be global or per request.