Free client seats on both sides, so compare what the seat opens
Adding clients costs nothing in either tool. Productive says so on its own pricing FAQ, and in Flowboard portal users live in a separate identity table from workspace members, with only members counted against the plan limit. What differs is the surface behind the login. A Flowboard client lands on a portal with their projects, their invoices, their forms, their booking pages, and the proofs waiting on their approval, with task, comment, and file visibility scoped per item. So what: the client seat is the same price in both, so the question is whether the work you need signed off is reachable from it.