The GT500 has a 7 speed dual clutch Tremec TR-9070 which is similar to the 8 speed Tremec TR-9080 in the C8 but in a longitudinal configuration. The auto shift will likely drive sales similar to how the 8 speed auto has in the Challenger and Charger Hellcat.
Definitely to each their own, different tastes, and different circumstances. The GT500 is going to give more of the front engine muscle car sound and visceral experience, but with a chassis of a modern sports car, giving a great GT car experience. The C8 is trying to compete with mid engine supercars
A friend of mine has a Mustang GT convertible because he has 2 kids, and if he isn't able to drive the kids in his car, he probably wouldn't keep his wife, plus they can also all go out in the Mustang so he isn't relegated to driving the boring family car.