Both, kinda? It's a title, technically, but as long as he holds the position I see him being referred to formally as "Ultra Magnus" as if it were his name. I suppose some of his closer associates might still call him Sentinel in private though.
Still haven't decided if "Ultra Magnus" as a whole is the title, or if just "Magnus" is the title and "Ultra" is a personal appellation, with former bearer of the title taking names like "Delta Magnus", "Nova Magnus", etc.