I believe Motor Trend and Car & Driver have both done Challenger/Mustang/Challenger comparisons and overall the Mustangs have kicked butt. Challengers got major props for their retro look, roomy interior, and great over the road manners, but flicking around corners, on & off the throttle and brakes, the Mustangs eat the Mopar up. That said, for my purposes I'd still buy the Challenger or even better a new Cuda, but I can definitely see why others would go with the Stang. My wife drove a 69 Mach 1 in high school and is seriously looking at getting a Mustang sometime this year.
Later, Jim