WSUS just deploys Windows Updates via the client’s built-in Windows Update agent.
SCCM is a fully-featured software distribution and deployment platform, which also has audit functions, and is very scalable. It has its own separate agent, and as @Ook says, it costs many beans per seat.
You can do Windows Updates with SCCM. The advantage is that it will forcibly apply the updates to clients on a regular basis (and during the SOE deployment process), but the downside is that it’s a full-time job to manage.