Would you say the 5d3's silent mode is more silent in comparison to it's normal shooting sound than the 1DX's?
I have both, the 5D MKIII's silent mode is VERY quiet, much more than the 1D X.
The silent mode in the 1D X doesn't really change the sound - it just separates the mirror return from the mirror-up and shutter sounds. When you press the shutter release, you hear the 'flop' up the mirror going up followed by the shutter opening and closing - then the mirror drops back into place when you release the shutter button. by separating the two parts of the sound, it seems to be a bit less intrusive.
However, the silent shooting modes on the 5D MKIII is far quieter.