It's in the same domain. The difference is that, as far as I can tell, Things seems to be about ongoing task management, and you still need to set due dates for your tasks. The purpose of PlanMyWeek is that proposes a date/time for tasks, on a (typically) week basis. It's meant to augment tools like Things, Calendars and Reminders.

The other difference, in Things (like everything else I've looked at) there's no separation of urgency and importance, just priority. The problem there is that, while the urgent and important map to the highest priority, if you constantly rank your tasks in this manner, you risk "starving" the important but not urgent tasks, until they become urgent.