Bumps are generally faster to render as they dont create any geo where as displacements do.

You would use a bump when you want small "displacements" and a displacement where you need a larger movement, as bumps over s cirtian value start to loose definition.

For example I would use a displacment map for the "M" but a bump for the plasticy texture.

Its also worth noting that you can daisy chain bump maps too so you can have multiple bumps attached to "one" channel so to speak, this lets you adjust the individual bump nodes for the files which affects the single materials bump property, pretty useful stuff! So for an example if you wanted to make a desk with scratches on, you owuld generall make one bump map and then connect that to the bump node, having to adjust it in PS to get it looking right, daisy chaning then you cna use one map for the actual wood grain and another map for the scratches and as each map has its own bump node and file you have much more adjustment off the bat.