you can use sccm to be both server and client patching. you can also use sccm for either client or server and use wsus for the other one.
Sccm uses wsus, well it uses the wsus api to get update information which it then uses to determine what needs to be patched.
mbsa doesnt need wsus to work, it just dials back to MS asks what updates are available and then looks at the machine or machines you've selected and then creates a list what it thinks should be installed.
wsus is a free and simple patch deployment tool which is limited in the reporting that you can get out of it out of the box and doesn't allow you to seperate out the restart from the deployment. We're using custom sql code to pull compliance reports from our customers wsus boxes.