What is the difference between publisher-subscriber vs. server-client?

edit

I ended up getting the answer to my questions by asking a friend:

Q: what is the difference?

A: Pubsub tends to be for different purposes: you subscribe to something, process it and publish it. Usually at a fairly fixed rate. You can subscribe to multiple things and publish multiple things. Multiple other nodes can subscribe to the same topic. You can even have multiple nodes publishing to the same topic.

Server client tends to be more request response. Ros can also do that, see Ros services or Ros action server. In client server a client makes an explicit request for some specific data.

Q: in which scenario would I use one over the other?

A: You can do a request like structure in pubsub, but that's not really the aim. Pubsub is super flexible in that sense. But if you need to process specific requests from clients pubsub gets tricky, because all topics are global. For example, in pubsub, if client a and b make a request and you process them and output two responses, how do they know which one was for a and which one for b? That is the benefit of server client over pubsub: server client makes explicit requests, while pubsub makes information global.