The main difference is that the Create PDF task doesn't have any user-definable configuration. It creates a PDF of a default quality, with default settings, no protection, etc. Also, Create PDF is an action task that's syncroneous to your process, meaning the rest of your process has to wait for the PDF to be created before continuing and grabbing another input.
The Image task, on the other hand, sends its data to the Image Services, which can process asyncroneously and in multiple instances (which can be configured in the PlanetPRess Workflow preferences, under the Messenger section, "PlanetPress Alambic" instances). Also, Image has your graphics compression settings, security settings, PDF metadata, etc.
Oh, and PlanetPress Image can output to other formats than PDF too, including JPG and TIF amongst others. Create PDF, obviously, only does PDF.
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