I have the VS825SD and really like it. (The VS825C is color only, SD does not mean SD slot)
The SD uses a better Sony camera which captures a better image with less light in the water, and it comes with the $50 camera panner included in the box.
The SD also lets you set the camera to color or to black/white, to fine tune to the viewing conditions. The color-only cameras gray-out or brown-out as the light conditions darken.
The SD has a new monitor which is 3x brighter, so you can easily see it in daylight conditions, either on the ice or in a boat.
And the SD has on-screen information including the camera direction, depth and water temp, the previous model doesn't have any of that info.