The CZ conversion is going to be (at least mechanicly) more accurate due to the sights and barrel being one piece.
Look
here at the difference between the standard and target versions of AA's 1911 conversions to get a picture of what I'm talking about.
That said the Witness combo is hard to beat economicly. If your not going to shoot competativly I'm sure the Witness 22 is good enough to kill soda cans.
ETA most of the conversions like the Witness will require the hotter 22 fodder to cycle right