Between the 468MGRC4 and 488RC4, the difference is notable. The tightening of the Hydrostatic head is very smooth and applies even pressure which makes the camera stay in the same exact position.
With the standard head, there is a slight drift in the order of one or two degrees as you tighten. When I purchased my ballhead I got the standard and eventually learned to adjust by slightly overshooting the vertical angle of the camera. I do it instinctively now but I wish I bought the hyrdrostatic head which I was unable to afford at the time.
As with everything, this matters only if it is important to you. Personally I am a maniac about composition and use my 100% coverage viewfinder to its fullest with rarely room for a shift (fill the frame I was told :) of even one degree. My primary cameras can also correct this by shifting their sensor, so if I end up not perfectly aligned, I can fix it without moving the head.