I've only had the Mazda for 3 days, so it's a little hard to directly compare. There is no getting around the fact that the 6 is slower than the Kia. The Kia does have about 100hp more, so it should be. The big difference is in steering feel and suspension calibration. Mazda just feels better.
My issues with the Kia is that it seemed to be ageing faster than other cars I've owned.
The seat design mixes leather and fabric, where the fabric meets the leather was starting to wear. It looks like the seat would be torn before the car was 5 years old. The Mazda does seem to have better interior materials.
The top of the steering wheel was starting to get weather damaged - seems to be a common issue with the Optimas.
The Kia had a driver's door rattle - another common issue with the Optima. The Mazda does seem to have a more solid door sound when closing.
The Optima required constant steering inputs when driving on the highway - which is also a common issue with the Optima. My Optima also had the pull to the left under power that some others had. The alignment spec of the Kia must be set for fuel economy instead of steering feel.
The front suspension was soft and the dampening didn't seem well sorted. The front of the car would almost bottom out and bounce on moderate dips.
The power of the Kia also didn't seem to be consistent. Sometimes the car would seem very fast, other times it would seem like the car was down on power. Could have been the ECU dialing back timing or the waste gate. I always ran 93 octane gas, so I don't know what the issue was.
I had a Snow White Pearl paint on the Kia. The rear bumper paint did not match the body. This was only noticeable in certain light conditions. If I would have noticed this before, I would have chosen a different color or a different car.
Other little differences I've noticed is things like the front doors on the Mazda seems a little shorter than the Kia. I'm 6'4", so the seat is far back. My shoulder in the Mazda is even with the B pillar. In the Kia, I was sitting a little ahead of the B pillar. Not a real issue, just something to get used to . The Kia's trunk is bigger and the back seat may have more leg and headroom. My kids are young so that doesn't matter much.