The main difference between SCCM & Essentails is that Essentials supports up to 500 clients and 30 servers. That's the limit, if your organization is bigger than that go for SCCM. Essentials don't include support for OS Deployment but you can solve that by using MS Deployment Toolkit 2008, which is a free product and use it side by side. Essentials also include features from Operations Manager for monitoring of servers