There's no single answer to whether a Camaro or Mustang is faster. It depends entirely on the specific model year and trim level of each car. Both manufacturers offer a wide range of models, from economical base models to high-performance versions with significantly different engines and capabilities. A high-performance Mustang Shelby GT500 will almost certainly be faster than a base model Camaro, for example.
To get a meaningful comparison, you need to specify the exact Camaro and Mustang models you're interested in.