Which would be a better option; tempo tasks or portal tasks? We are doing a pilo

Definitely use Tempo tasks. When you enable Tempo on a task form, that form is visible on a variety of browsers (Chrome, Safari, Firefox, IE) plus a variety of mobile devices (Android, iOS, BB). Apps view of the task still works, but is limited to Firefox and IE. Regarding the reassign task, we recommend you build that step into the process. Add a Reassign button followed by a gateway followed by a chained task which gives the user reassignment options.

Read more about mobile: www.appian.com/.../mobile-access.jsp

In the documentation it is said - "forum.appian.com/.../Tasks" Rejecting and re-assigning tasks can only be done through the Portal.

My use case is if a resource is OOO then his manager should be able to see all the tasks assigned to a group and reassign accordingly. This can be done out of the box in portal? How do i achieve the same functionality for managers in the tempo

Reassignment is not available directly in the Tempo Task view. That is why we currently recommend you design the reassignment step into the process.