Biggest difference from enterprise point-of-view is that Monitor gives you ability to monitor multiple VirtualCenters. As you know VirtualCenter has pretty limited capacity, and many of our customers have more than one VC.
From feature perspective, VirtualCenter does not have most of the features listed in the What's New for Monitor 3.0 Beta (except the last two - but again, even for these two VC can't provide you similar functionality if you have more than one VC). Please check the What's New document out, and if you have any questions about specific features, just ask us.
From usability perspective, VirtualCenter is simply not a monitoring application, and as your virtual environment grows you will soon find that using it as such simply does not work well. We are hearing this a lot from our larger customers. Basically, VC UI is strongly focused on management, rather than monitoring tasks.
From VI performance perspective, using Monitor for monitoring tasks offloads VirtualCenter significantly. This again comes down to very common customers' concern about VirtualCenter performance. Our customers wanted to offload VirtualCenter from extra tasks like monitoring, this was one of the reasons why we started to build Monitor initially.