Cadillac or Chevy
Zenoss Enterprise and OpenNMS are both very capable and flexible network monitoring systems, and they present great values when compared with big applications from the likes of IBM, CA, and HP. These two network monitoring systems have a long list of features, and IT staff looking for a monitoring system should carefully consider their needs, then research both OpenNMS and Zenoss to determine which is a better fit for their organization.
Zenoss Enterprise clearly has a more developed feature set, including ACLs, VMware vSphere 4 support, automatic root-cause analysis, distributed collectors, collapsible network diagrams, and software inventories. However, those features come with a significant price as compared to OpenNMS, which is free, open source (GPL) software. Further, OpenNMS provides support at a flat rate; the cost is not tied to the number of network devices your company will monitor. Because Zenoss Enterprise is sold as a subscription rather than an outright purchase, companies will have to commit to budgeting the cost of Zenoss each year. And the cost of your subscription will increase if you add more network devices to be monitored.
If you need the extra features of Zenoss Enterprise and can allocate the annual budget for it, then Zenoss will be a terrific purchase for your company. You will have no trouble selling the idea of using Zenoss to the company higher-ups when you stack it up against HP, IBM, or CA. If you have a limited budget or do not need the extra features of Zenoss Enterprise, then OpenNMS is a real winner, and support is easy on the bottom line.
Furthermore, if you are not in hurry to deploy a new network monitoring system right now, then it would be prudent to wait a month or two to see how the version 1.8 release from OpenNMS turns out. If OpenNMS delivers on the planned features, then a number of the software's shortcomings (including VMware vSphere 4 support) will be addressed and OpenNMS will become a more suitable fit for some companies.