I've checked both shotguns out. The FP6 is about $100 - $150 more expensive than the 870 HD here in Socal.
The one thing I was not happy about w/ the FP6 was that the heatshield seemed to have a gap between the barrel. Plus although the CF looks pretty damn cool, it doesn't seem durable. I've read somewhere also that the CF finish isn't real CF of course, and its just a sticker thats applied to the shotgun. Looses points in my book.
The 870 HD. Cheap finish, but everything else I liked. The fit was good as usual for every other 870. I liked the look of both, but the 870 has the edge cuz its what I'm most used to.
In the end, I'd take the 870 and pocket the extra cash