The diesel will get better mileage on the highway, but around town it's going to be a wash at best.
This. I find that the advancement of ICEs has made the diesels in current form somewhat moot unless the only driving you do is pure highway.
I'd like to see more R&D on diesels to make them the efficient powerplant that they can/should be. There's simply been more focus on petrol for some reason.
Even Mazda's incredible 2.2D is only going to have 'some' benefits that will not likely outweigh its costs. To sell both the 2.5G and the 2.2D side-by-side is to leech sales. If the Mazda6 was purely sold as a 2.2D (no sky-G available), it would be a niche product that would have some real sales if sold at the same pricing as it current is at. The problem is, for a $2-3k premium over the gas version, and similar performance, and only the possibility of 'some' fuel savings, the diesel doesn't make a ton of sense.
It could, if done right (as mentioned above), but then there's some people who say, "eww, diesel" and wouldn't even shop for one.
Damned if you do, damned if you don't.
The Fusion S is significantly cheaper than the Mazda 6 GX. I just went to Ford.ca and built a SE with technology and appearance pkgs for $1800 less than the GX. The Fusion is also better equipped at that price...
...and there you have it, folks, why the Fusion sells more than the Mazda6.