VMware had tried hard to push their existing customer to switch over to ESXi version since vsphere 4. With the up coming vSphere 5, there will no longer provide esx service console as it does in the previous version. vSphere 5 will only come with ESXi edition with leave the users no choice by adopting to it.

For users who plan for an upgrade in the near future or any new deployment, I will suggest you to deploy all your ESX host with ESXi for the ease of upgrade or migration. The upgrade process is pretty straight forward from version 4 to version 5 without any big hassle.