Photomatix Essentials is particularly easy to use and intended for photographers new to HDR.
Photomatix Pro offers more options for adjusting HDR photos and includes advanced features such as batch processing and selective deghosting. It also offers presets intended for real-estate photography as well as a Plugin for Lightroom.
Photomatix Essentials and Pro share the same internal HDR processing engines. However, their interfaces differ. Photomatix Essentials focuses on simplicity, providing an easy-to-use introduction to HDR.
If you buy a license of Photomatix Essentials, you can upgrade to Photomatix Pro later on for the price difference between the two products.