My brother has the Mazda 6 sedan.
The only difference i think there is that, the trunk looks slightly different.
For example the Mazda 3 Sedan(4 door) and Hatch(5 door). You can tell a difference.
the back window opens with the trunk.
Here is the Sedan
I don't believe there is any difference in performance wise.
But I still love my Mazda 3 more.