## Gengar ##
Gengar, overall, would be a better choice.
Competitively, Porygon Z is UUBL, and Gengar is OU in Smogon.
Both are fast special attackers.
Stats: Porygon Z has better/equal stats in all but speed.
Typing: Gengar is a ghost-poison, while Porygon Z is a normal type.
Ability: Porygon Z has download, an amazing ability that either buffs attack or special attack depending on the defense and special defense of the opposing Pokemon. Gengar has levitate, making it immune to ground, a type that would otherwise be super effective.
Gengar has speed that makes it able to defeat many Pokemon before it gets defeated itself. Also, it has a typing that makes it able to cover some of the weaknesses of your team, such as ground for victreebel, scizor, and rhyperior, fighting for rhyperior and psychic (offensively) for victreebel. Normal is not a good type for supporting a team.
Gengar has a vast movepool that supports your team both offensively and defensively, with sludge bomb, pain split, etc. Porygon Z, while having an amazing offensive movepool, is useless for defense, even though it has decent defensive stats.
Pick Gengar. It is an easier evolution to get and is better in most other ways.