Diferent colors, no neck, TRI attack makes more sence on it, and that thing you call horn I see a antenna, like many electronic things has, plus two cool new abilities! Also PZ lost completely the duck (or rubber duck) style.
So it´s not just a diferent Porygon2, it´s a cool new Pokémon =D 2nd favorite in my list ;D
Ah add an interesting fact, they weren´t uncreative on naming it Porygon3 (like Sony did with Playstation xD).
Z is like a 2 (If you didn´t understand write a Z and a 2 next each other), but bigger and sharper.