Depends on the camera and sensor. Both my 7D and my 5DII have auto sensor cleaning. But I've never - not once - had to manually clean my 7D, whereas I have to clean my 5DII every 3-4 months. Approximately equivalent lens changes, too. So, IMO the FF sensor attracts and holds more dust. The 5DIII has the same cleaning system as the 5DII - Canon seems to think it has its limitations, because they put a newer, better system in the 1D X (looking forward to that!).