Porygon Z also makes Porygon the only pokemon in the game with a two trade chain evolution,the first one requires Upgrade (Porygon 2) and the 3rd form requires the dubious disc.
and besides that Porygon Z has adds to the uniqueness of this pokemon...debugging time!
as far as i can tell from his moveset,he gets the addition of nasty plot and embargo as options,but replaces Trace with Adaptability (but still has the option of download)
Adaptability improves STAB
strange how i seem to like to talk about a pokemon i never use >.>