Having driven both here's how I see it:
Performance: Basically equal in acceleration, but the Ford handles much better even though it's heavier. For some reason honda decided handling wasn't a thing for their V6 models (part of the reason I own an altima v6 now after having a 4 cyl accord). Seriously, you can try to make a turn at speed, but it seems like the car just hates every part of it. The fusion isn't a sports car, but I'd call its handling confident, and you're happy you're on a twisty road instead of frustrated. I'd say my Altima handles noticeably better than the Accord, and the Fusion Sport is noticeably better than my Altima. Honda's transmission shifts better on its own I think, but gives you no control over choosing your own gears if I recall correctly.
Ride: Pretty equal to me. If anything the Fusion seems like a more controlled ride, without much harshness.
Space: The accord has more of a boxy shape, and has more headroom and legroom for adults in the back. I'm 6'1" and felt slightly cramped in the fusion rear behind my seating position, but not unbearable by any means. The smaller windows in the fusion make it feel a bit more cramped sometimes, but probably something you'd get used to easily. Trunk space is about equal (good for both cars).
Interior: Both are fine for the price range, but won't be confused with a Mercedes. As far as materials and design, I'd give a slight nod to the Accord, but not enough to really make a huge difference personally. I actually really like the fusion sport suede seats because they stay pretty cool on hot days. One big factor is the fusion is MUCH quieter inside than the Honda. I was amazed at how much road noise was in the new accord. I'd say my Altima is a level quieter than the Accord, and the Fusion is a whole level quieter than the Nissan.
Cost to own: Advantage - Honda. most V6 owners report similar mileage to the 4 cylinders, and average around 27-28 mpg vs the Fusion sport's 20. Also, their v6 is not direct injection, has no turbos, and it's only driving 2 wheels, making for a much simpler overall design from a company that is known for reliability. I'm not saying the Fusion will be unreliable, but odds are the Honda will last longer and be cheaper to fix.
Other notes:
AWD is nice even in just rain to know you can make the merge, etc.
While the Fusion sport doesn't get great gas mileage, gas is cheap, and you can run regular if you need to.
US carmakers usually offer better incentives than Japanese ones.
Personally if I were buying a new family sedan for around $30k, the fusion would be my choice. It's slightly heavier and slower than I was hoping, but the difference in handling, AWD traction, and sense that it's slightly special put it in the front of its class in my opinion. Obviously not everyone has the same priorities, and a $4k difference in price can be a big deal too.
