Plesk has a better user interface and a better reseller interface .. from our clients point of vue (from the feedback I get), I would say that Ensim clients report more problems than Plesk clients. But maybe I simply do not have enough feedback from Plesk clients ... we started offering Ensim before Plesk. I personally do not like the design of Ensim's interface, I think Plesk is well designed.
The best thing to do is to try the demos .. both Ensim and Plesk have online demos on their web site. I think you need to see what functionalities you need and try the demos to see which one best fit your needs.