Clip will always obliterate anything oustide the clipped area. Good for keeping what you want and getting ride of what you don't.
Mask will obscure the image to various degrees. Anything covered by 100% black will not show through the mask. Anything covered by 100% white will show through the mask completely. If you do shades of grey, the image will look faded. Good for creating effects. I've used it to make things look like they're out in the distance or to obsuce something that's supposed to look like a reflection. There are, undoubtely, other uses. I saw a tut on the inkscape site that use mask and blur to great the illusion of depth on a pen.
~A