FWIW, I experimented a bit with clear & diffused LEDs of the same color, and although one was slightly brighter than the other (the diffused one, surprisingly), under large keys (such as Capslock) both failed to lit the entire legend.
What matters is the angle of view of the LED : a narrow angle (30° for instance) will not allow the entire legend to be lit; a wider angle (up to 120°) will. I think that diffused LEDs often have wider angles of view than clear LEDs, but it's not always the case. So, don't buy blindly, always look at the datasheet.