Why not both? Some servers can run open federation, some can run allowlist-only, some can run in quarantine-first mode, and over time I'm sure we'll see shared lists, reputation signals, and trusted upstream servers to help manage the onboarding/allowing.
"Disallow all, but allow all servers already allowed by x, y and z" is one way to approach.
Almost none of the asks I've seen are either/or propositions, they are generally admin options to enable or not.