Goliaths have never been particularly good at ground combat actually, so Thor pushes would far more deadly. Rushing in with Goliaths would be like rushing in after landing Vikings outside of the enemy base.
Vikings are also better for contains since the opponent cannot force friendly fire on them from Siege Tanks, nor target them down with the same ground-only units that make up the bulk of most armies. Vikings also aren’t limited by terrain, which is a big factor in all contains.