The Anker ones, if I recall correctly, are larger capacity while still fitting within the original cavity (won't bulk out the phone and require a new cover like traditional extended batteries).
But I go with the cheap knock offs on ebay. In fact, my last 3 or 4 phones, I would buy a kit with 2 spare batteries + standalone charger (so one battery is always on tap and ready to go) for less than a single Anker off Amazon. Capacity was usually as good if not better than OEM.