Photomatix Pro offers more options for editing HDR photos and includes advanced features, such as selective ghost removal, blending with a brush tool and advanced color settings.
Photomatix Pro has presets designed for real estate photography, a plugin for Lightroom and batch processing (an appreciated time saver when you want to process many photos).
Photomatix Essentials also offers powerful HDR editing options, but in a simplified user interface. It will help you learn about HDR photography without overwhelming you with advanced choices, while sharing the same internal processing engine as Photomatix Pro.
If you buy a license for Photomatix Essentials, you can upgrade to Photomatix Pro later on for the price difference between the two products.