I prefer centralized, so that the competencies are under the same manager and you can collaborate on skills building with-in the team. From there you work out into the projects to deliver testing on the long or short run. This is what I see for other IT skills too, a team for developers, a team for Citrix, a team for security etc.
Others dread the “test centers” and successfully establish “loose communities” (Spotify model). Though not under the same manager it can be hard to prioritize shared needs/skills/conferences - as the focus is the delivery teams. If there is less need for a testing specialist in the team, this might be the route.
It seems though like ebb and flow, organisations centralize and decentralize over the years.