These are similar software, based on the same core modules, but they have a few important differences.
The Windows PBX is a simplified version of the Softswitch with easy install and management. The main difference between the Softswitch and the Windows PBX are the followings:
Except the above modules, the Windows PBX licensing model and features levels are similar to the Softswitch as desribed here.
The Windows PBX can be upgraded at any time to the Softswitch.