They're very different products. UniFi is a controller-based system that is designed for greenfield deployments (all UniFi hardware, custom VLAN setup for it). It requires a rather, um, unique, VLAN configuration - it requires it's MANAGEMENT be untagged frames. Strange, and not really correct (management should occur on it's own management VLAN). However, deployed as intended, UniFi is a very cheap functional controller-based system.
EnGenius makes conventional web-managed controllerless AP's. They're cheaper, easier to deploy on an existing network (no massive configuration changes required), but harder to manage especially once you get to the 10+ AP's level.
The EnGenius EAP600 is, within these massive differences, the most equivalent to the UniFi AP PRO.