Neither of them are on the NatDex UU VR, but I'd say Porygon2 would be better since it has a niche as a Trick Room setter. PorygonZ is outclassed by Victini and Jirachi as Pokemon who use a Z move to become a scary breaker/sweeper.
commented 3 days ago by -RisingManectric-
Although outclassed by Chansey and Blissey for a Normal-type tank, it has a pretty good niche as a Trick Room setter with its easily outclassed speed, however can outspeed some other Pokemon.
Role: Trick Room setter
252 HP, 252 DEF/SP.DEF
Ability: Analytic
Item: Eviolite
Sassy or Relaxed nature
Recover (Good recovery if you want to stall.)
Trick Room
Tri Attack (pairs well with Analytic, STAB)
Thunder Wave/ Toxic (Thunder Wave cripples opponents, Toxic for stall.)