Hello,
you are talking about the Activiti Designer / Modeller plugin for Eclipse. Basically, an Alfresco User Task and an Activiti User Task are the same thing from a BPMN 2.0 / Activiti point-of-view. The only major difference is in how the modeller plugin handles those elements. For an Alfresco User Task, the formKey handling is slightly different as the modeller provides a selection utility based on a workspace preference setting that lists all available task types. Other than that, I have not found significant differences.
In my personal use, I have come to completely disregard any special Alfresco support the modeller has to offer and always use the base Activiti elements. Once you leave the beaten path of the tutorials, howtos and wiki documentation, you quickly find yourself at the limits of what the built-in special elements are able to do, and only find the necessary freedom / flexibility / robustness with the standard ones.
Regards
Axel
