I'd pick the Shelby over the Charger any day of the week if I were the muscle car type. Even back in the days when I didn't know much about cars, I knew the Mustang. Route 66 and the mustang have some serious synergy that I can't believe a charger could compete with. I agree with Paul; the Charger for the weekdays, and the Mustang for the Weekends.
Still, I wouldn't buy either car. I like to fantasize about owning a muscle car, but I'd never actually buy one. I'd never take it to the track, and I'd get exhausted from being such a douche-bag while stop light racing everyone.
I'm the cruising type, I sometimes don't even match the speed limit. Which is why, when I drive 66, I'm taking something soft, and convertible. But I read Tim Falconer had an OK time in his old Maxima. So, I'm sure the trip is worth it in just about anything that will hold together.