I'm currently a Mazda6 GT owner, and I'm trading it in for a Stinger GT1 or GT2. I've test-driven the GT-Line, and honestly the GT-Line performs about the same as my 6, and my 6 is more comfortable to be in. If you are sticking with a 4 banger, I'd honestly go with the Mazda. Better bang for the buck, IMO. Mazda also has what appears to be quite solid resale.
With that said, there is absolutely no comparison between the V6 Stinger and my Mazda6. Completely different classes of vehicles, which I discovered after test-driving the GT2. Driving was actually fun in the V6 Stinger, while it was just a necessary action in the Mazda6. Obviously the gas mileage is a good bit better with the 4 banger, but the smile the V6 will put on your face as you zip in and out of traffic is more than worth it, IMO. And the Stinger just looks much better, but I realize looks are subjective.