WOW is instanced, but the instances are all hosted by Blizzard. Instances are basically a replication of the world state, and managed automatically depending on load. If you join a group, you will automatically switch to the instance those players are in.
In Destiny, some of the instances are hosted directly on the client. Depending where you are and what you are doing.
In Destiny, some of the instances are hosted directly on the client. Depending where you are and what you are doing.