Well you've got to identify your apples and your oranges here...
dithering in it's simplest definition is adding noise to your signal to smooth out truncation errors caused by changing bit rates. In a more complex discussion there are several different algorithems that will allow you to do this and avoid huge artifacts, this is where you need to decide which apple is related to what orange. I don' think you can make generalised statements comparing dithering processes without first identifying which algorithem you are talking about...you know apples to apples...
All dithering adds noise, I believe apogee is using UV22 which refers to the fact that almost all the noise added in their dither happens at 22khz (just out of earshot), this would explain a perceived difference in other dithers, but personally I would only use that dither after all other processing was done. There are differences in the way the dither algorithems sound compared to one another, but some of it is subjective.
Q:Why did the bass player break his window after he locked his keys in his car?
A:To get the drummer out.