I have used Helm before as an end-user, never as an administrator. I have never used Ensim before. These will be running on Windows 2003 Standard Edition (at either ThePlanet or EV1, we have not decided yet).
Any comments on either package as far as flexibility, power, and ease of use? There will be NO "end users" so that is of no concern if everything can be done via the administration panel.
I love Ensim pro for its simpliscity and out-of-the-box experience. Helm gives you a lot more control but in order to take advantage of that, you really need to know what you are doing. Takes a while to understand how everything works.