I don't own the W900A, so I can't speak for that, but I did buy an F8000 in March. I have very little complaints about the F8000 except for the input lag and a slim viewing angle. 30 to 40ms under PC mode is normal for LCD televisions, but Sony's new sets can reach 8ms. Unbelievable.
On the other hand, the F8000's picture quality is superb. Colors are sharp, blacks are deep, and it has near perfect uniformity. My set has no flashlighting, clouding nor banding issues. My Sony HX850, the predecessor to the W900A, had obvious vertical banding during camera pans that drove me to return it despite its otherwise wonderful 2D image. 3D is also quite excellent with a bright, flicker-free image. I disliked the 3D on the HX850, but it sounds like Sony did a lot to reduce crosstalk for the W900A. I expect it might perform as well in that regard as the F8000, which keeps crosstalk to a minimum. Finally, build quality on the F8000 is superb. Its bezel and even its stand are practically invisible; it's all screen.
Between those two sets, it's a tough choice. You're not going to go wrong with either one. If you do a lot of competitive gaming you may want to go with the W900A, though the F8000's input lag is still perfectly acceptable. Both sets will have to disable a majority of their picture options to get that low input lag, and that may be one area where the F8000 could pull slightly ahead. Even with most of its picture options unavailable in PC mode, uniformity was still perfect. I'm curious to know if the W900A retains a uniform picture when game mode is enabled. An owner here will have to test that and I'd love to know the answer.