What is the difference between user_id and account_id in the User object ?
Both are different and I'm looking for the unique ID of the User.

    {
      "items": [
        {
          "user_id": 2144246,
          "user_type": "registered",
          "creation_date": 1362659682,
          "display_name": "Tony",
          "profile_image": "https://i.sstatic.net/ZEhow.jpg?s=128&g=1",
          "reputation": 68,
          "reputation_change_day": 0,
          "reputation_change_week": 0,
          "reputation_change_month": 15,
          "reputation_change_quarter": 25,
          "reputation_change_year": 67,
          "last_access_date": 1377435062,
          "last_modified_date": 1377018698,
          "is_employee": false,
          "link": "http://stackoverflow.com/users/2144246/tony",
          "website_url": "",
          "account_id": 2417804,
          "badge_counts": {
            "gold": 0,
            "silver": 0,
            "bronze": 5
          }
        }
      ],
      "quota_remaining": 9999,
      "quota_max": 10000,
      "has_more": false
    }