I think the accuracy they quote is bogus also since the "umph" will most likey move the carrage/topslide a small amount greater than what they quote.
However I can see where the wedge type would be more repeatable.The front of the dovetail is fixed and rigid while the back is the moveable part.When you engage it the toolholder pulls back against and tight to the front fixed side of the dovetail which provides positive location.
The piston type on the otherhand has the holder being pushed out against both sides of a fixed dovetail,but because of the clearance needed to facilitate changing holders there is a possiblity of the holder cocking slightly askew,but only by a few thou.
I don't think this will matter a great deal thou.
I just need one more tool,just one!