These are completely different products. Comparing them is much like comparing a farm tractor to a bulldozer. They both have engines and love to play in the dirt, but the similarities don't go much deeper than that

SME is intended to provide services to a group of people, such as a business or school for example. As such it conveniently sets up user accounts and associated services, primarily for internal access. While it can host public services such as web and mail and ftp, that is not it's forte. It definitely does not excel at hosting web/mail/file services for a number of unrelated customers. On the other hand, the price is right. I host a couple dozen sites on a system based on SME server, but it is seriously hacked and a far cry from a stock system. It is also a pain in the arse when it comes to managing email accounts.
CPanel (and Ensim and the other "control panel" systems) are designed to run web hosting businesses. As such they make setting up web sites and email accounts and ftp logins and ssh access a breeze. They are built to host multiple domains, to provide easy SSL setups, and so forth. They would absolutely suck if pressed into service to run the network for a small/medium sized group.
Bottom line:
If you want to host web sites etc, get a control panel app like CPanel or Ensim.
If you want to run a network for a small/medium group, get an appliance like SME Server or ClarkConnect.