Hi

first a disclaimer: I wrote uSync

It sort of depends on how you are going work on your site. uSync and Courier were set up for two very different workflows.

Courier, will allow you to deploy stuff from within the umbraco environment, and as you say lets you target things like prod/staging. Courier is very much push changes around.

uSync was build from a source control / multiple developers point of view, so it tracks changes all the time. and it leaves it up to you as to how you propagate changes between your environments.

In it's default state, uSync is always writing changes to disk, if you then source control them, deploy them to other sites, then uSync at the other end will import them (on start, or when you press the buttons in the dashboard).

For development - the idea is that changes just 'happen', changes between dev machines go into source control and get synced on all local installs. How you deploy changes to live depends on your own deployment processes.

uSync is very flexible in how it imports/exports stuff, so you can play with all of this stuff to suit your own development practices. You can do CI with the right combination of build and deployment tools, but uSync doesn't get involved in how you do that.

Courier is used inside umbraco as a service to do this type of continuous deployment - with how much additional config/code i do not know, so in theory it's also possible to put this workflow in with it? but i am not sure.