* A Mustang GT (4.6L V8) would likely beat a Charger SE (2.7L V6) or even a base Charger R/T (3.5L V6). The Mustang GT's V8 provides significantly more horsepower and torque.
* A Mustang V6 would likely *lose* to a Charger R/T (5.7L Hemi V8). The Hemi V8 in the Charger R/T offers substantially more power.
* A Mustang GT versus a Charger R/T is a closer race. It would likely come down to the drivers' skill, the condition of the cars, and the specific track or road conditions.
In short, there's no definitive answer without knowing the exact engine and trim levels of both the Mustang and Charger involved.