Both manufacturers offered a wide range of engines and packages in 2012, from base V6 models to high-performance GT and SS versions. The Camaro SS with its LS3 V8 typically out-accelerated the Mustang GT with its Coyote V8. However, the difference wasn't always dramatic, and track testing results varied slightly depending on conditions and testing methods.
To get a definitive answer, you'd need to specify the exact trim level (e.g., Mustang GT vs. Camaro SS, Mustang V6 vs. Camaro V6) of each car.