There's no difference, other than the Displacement Modifier can have its settings adjusted independent of global displacement - so if you want your scene to be in Screen space for displacement for efficiency, but one object really really has to have a preset level of displacement detail no matter how far away it is, you can use the Displacement Modifier on this "hero" object and set it to World with whatever resolution it needs, separate from the overall scene. So you can view displacement in a material as global, then override only where needed with the Corona Displacement Modifier to increase detail (or decrease it, etc. etc.!)
(on a separate note, the Max Displace modifier is one that can show up in the viewport, as it's not a render-time displacement, just in case you need to see that displacement in the viewport)
