The verdict
If you run large, deadline-driven campaigns with many dependencies, pick ClickUp. If your team writes and plans more than it tracks, and already lives in Notion, pick Notion AI.
I ran ClickUp at my own agency for about a year and then moved the team back to Notion, because we wrote and planned more than we tracked. That is my own situation, not a rule. If I managed campaigns with many dependencies, I would stay on ClickUp. Notion AI is not sold as a separate add-on any more. It comes with the Business and Enterprise plans, so the real price is moving the whole workspace up a tier.