My brother has a 2018 GT (6 speed manual) that I've driven extensively.
I love it. The engine noise, the handling, the acceleration, the manual transmission. Ford nailed everything a Mustang should be.
Even with the manual transmission it'd be close in a 0-60 vs. my Stinger. It all comes down to the one shift the Mustang driver has to make.
The Stinger wins out in the practicality department, in terms of cargo space and seating four people comfortably. Better gas mileage. More luxury features. The lane assist in the Mustang is downright awful compared to my Stinger. I'm still trying to figure out the sound systems, but despite the Mustang having an overall lower wattage, it sounds incredible. Once I get mine dialed in it should be equivalent or better, but currently I prefer the one in the Mustang.
I don't have any regrets with my Stinger, but they're both amazing cars.