ESX has a Service Console (a separate RHEL virtual machine to run many management functions).
ESXi no longer has this separate Service Console. The management functions of the Service Console still exist but from a POSIX based machine (busybox) which is compiled within the kernel of ESXi. The result is a smaller amount of code, and no web server or firewall built in - those are the main differences. The vSphere Client will generate the web pages so you can still manage ESXi in the same way as ESX. vCenter can manage ESXi. As already mentioned there are also CLI and scripting interfaces available to use with ESXi (vCLI, vMA, PowerCLI).
As also mentioned, ESXi is the future, so go with it now and even start managing existing ESX installations with ESXi tools like the vMA (it will ease the pain of migrations for your customers in the future).