The S6400 has the following Quick Effects: Pop,Super Vivid, Toy camera effect 1, Toy camera effect 2, High key, Low key, Cross process (red), Cross process (yellow), Cross process (green), Cross process (blue), Soft, Fisheye, Cross screen, Miniature effect, High-contrast monochrome, Sepia, Cyanotype and Selective Color.
The S800C does not have a Quick Effects option but can apply the following special effects filters: Soft, Sepia, High-contrast monochrome, High key, Low key. It is also able to apply these filters after a picture is taken: Vivid, Black and white, Sepia, Cyanotype, Soft, Selective Color, Cross-Screen, Fisheye and Miniature effect.
Both the S6400 and S800C have CMOS sensors and ISO ranges of 125-3200 for excellent low-light performance. They also both have Night Portrait, Night Landscape, Fireworks and Dusk/Dawn Scene modes which will automatically optimize the camera's settings for the selected scene.
The best way to compare these 2 cameras is to use the Compare option on Nikonusa.com. Go to the below page, check off the cameras you wish to compare then click "Compare Now" to see the specs for these cameras side-by-side: http://www.nikonusa.com/en/Nikon-Pr...