Terrantula vs. Backstage / Port
Different unit of analysis. Backstage, Port, and the other internal developer portals catalog services — ownership, metadata, scorecards, golden paths for app developers. Terrantula models live infrastructure — entities with lifecycle states, cells with capacity ceilings, relationships with cardinality, derived from your Terraform state.
A catalog entry doesn't have a tenant-count, a "max 50 per cluster" constraint, or a placement policy. A Terrantula entity doesn't have a scorecard. The portals serve app developers self-serving through a platform team's templates; Terrantula serves the platform team running the stamped infrastructure underneath.
They coexist. Plenty of teams run a portal for their services and need a fleet layer for their per-tenant or per-account infrastructure — the portal never answers "where does the next tenant land?"
The honest overlap: Port's entity graph is flexible enough that a determined team could model a fleet inside it. What it lacks structurally is Terraform-state-derived ground truth, capacity-aware placement, and changes emitted as pull requests your CI applies (why that boundary matters). Terrantula is fleet-shaped on day one; Port can be bent toward it. If you're already deep in Port and not hitting walls, staying put is a reasonable answer.
Not for you if what you want is a service catalog, scorecards, or app-developer self-service — that's genuinely the portals' job.