2009 2.3L Mazda 3 hatchback.
1) The seat was much more comfortable. Manual versus power, interesting, but so much better.
2) Visibility was lesser. I felt more cocooned inside the car, lower down, further back, restricted. It was very noticeable off the right front corner, and to a lesser extent, left front corner. Afterwards, I concluded the area right under the wing mirrors is 2" higher off the ground for the 3, and forwards of that, the 6's nose drops off quickly, the 3's nose stays taller. It creates a big tall blind area, and it bothered me. Checking view behind was a little more challenging, the interior mirror was more obstructed, and I didn't go on highway to confirm lanes.
3) The handling was fucking excellent. It started up, threw it in D, and whipped out of the lot. The 6's handling and my sense of its location surprised me when I first drove one, but this was on a whole other level. The 3 had a poorer sense of its location, but such tight handling.
4) The motor was the same as the 6, and so, extremely disappointing. Torque steer was found under hard acceleration on a right hand turn, either that, or I just overturned, either is possible.