Alolan Raichu is much better. Not only is it faster, with higher offensive and defensive stats, it also gets access to Psychic. The only thing that makes Pikachu better than Raichu is the Light Ball, which cannot be held if you use Pikainum-Z. Besides, Alolan Raichu has a Z-Move of its own — Stoked Sparksurfer — with the added effect of always paralyzing the opponent.
Another thing — if you're talking about Pikanium-Z and not the Electrium-Z, it only works if Pikachu knows Volt Tackle, which has to be bred onto it. So, for in-game purposes, Alolan Raichu is much better than Pikachu.