I have bags from both companies, a -30 or so WM bag that's now discontinued but is analogous to a Puma SDL and a 20 FF swallow. I've spent about sixty nights in the WM bag, and about twenty in the FF bag.
I'd say that the WM bag was just slightly more finished looking at first glance, but I don't think the build quality was any different. The FF bag (and the FF parka that I have) are built along a no nonsense kind of ethic and their tailoring is a bit peculiar. FF has some seams that must be really hard to do -- and remember, I design and build backpacks for a living -- and I think that these seams make the FF bags look a little less polished at first glance. However, I think (i've never designed a down bag or a parka, i've just used them, enjoyed them and destroyed them) that the FF seams might be why the stuff seems to fit me better than the WM bag I have does.
I would sort of like say that the FF bag seems warmer for its rating than the WM did, but then I think that comparing winter camping after ice climbing to sleeping under the stars in the summer is stupid, so I'd better not go there. I'm 6'5", about 200 pounds, and I sleep pretty cold.
All in all, I'd say that if I had the time, I would order a custom FF bag or quilt for myself, but if I needed to get it off the rack, I'd most probably go with WM.
edit: i try in the practice area, and i still mess up...jeez