Not Exists vs Not In: efficiency

Question

I've always been under the assumption that not exists is the way to go instead of using a not in condition. However, I doing a comparison on a query I've been using, I noticed that the execution for the Not In condition actually appears to be faster. Any insight into why this could be the case, or if I've just made a horrible assumption up until this point, would be greatly appreciated!

QUERY 1:

SELECT DISTINCT 
a.SFAccountID, a.SLXID, a.Name FROM [dbo].[Salesforce_Accounts] a WITH(NOLOCK)
JOIN  _SLX_AccountChannel b WITH(NOLOCK)
ON a.SLXID = b.ACCOUNTID
JOIN [dbo].[Salesforce_Contacts] c WITH(NOLOCK)
ON a.SFAccountID = c.SFAccountID
WHERE b.STATUS IN ('Active','Customer', 'Current')
AND c.Primary__C = 0
AND NOT EXISTS
(
SELECT 1 FROM [dbo].[Salesforce_Contacts] c2 WITH(NOLOCK)
WHERE a.SFAccountID = c2.SFAccountID
AND c2.Primary__c = 1
);

QUERY 2:

SELECT   
DISTINCT
a.SFAccountID FROM [dbo].[Salesforce_Accounts] a WITH(NOLOCK)
JOIN  _SLX_AccountChannel b WITH(NOLOCK)
ON a.SLXID = b.ACCOUNTID
JOIN [dbo].[Salesforce_Contacts] c WITH(NOLOCK) 
ON a.SFAccountID = c.SFAccountID
WHERE b.STATUS IN ('Active','Customer', 'Current')
AND c.Primary__C = 0
AND a.SFAccountID NOT IN (SELECT SFAccountID FROM [dbo].[Salesforce_Contacts] WHERE Primary__c = 1 AND SFAccountID IS NOT NULL);

Actual Execution plan for Query 1:

Actual Execution plan for Query 2:

TIME/IO STATISTICS:

Query #1 (using not exists):

SQL Server parse and compile time: 
   CPU time = 0 ms, elapsed time = 0 ms.

SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 0 ms.
SQL Server parse and compile time: 
   CPU time = 532 ms, elapsed time = 533 ms.
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Salesforce_Contacts'. Scan count 2, logical reads 3078, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'INFORMATION'. Scan count 1, logical reads 691, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'ACCOUNT'. Scan count 4, logical reads 567, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Salesforce_Accounts'. Scan count 1, logical reads 680, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
   CPU time = 250 ms,  elapsed time = 271 ms.
SQL Server parse and compile time: 
   CPU time = 0 ms, elapsed time = 0 ms.

SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 0 ms.

Query #2 (using Not In):

SQL Server parse and compile time: 
   CPU time = 0 ms, elapsed time = 0 ms.

SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 0 ms.
SQL Server parse and compile time: 
   CPU time = 500 ms, elapsed time = 500 ms.
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Salesforce_Contacts'. Scan count 2, logical reads 3079, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'INFORMATION'. Scan count 1, logical reads 691, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'ACCOUNT'. Scan count 4, logical reads 567, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Salesforce_Accounts'. Scan count 1, logical reads 680, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
   CPU time = 157 ms,  elapsed time = 166 ms.
SQL Server parse and compile time: 
   CPU time = 0 ms, elapsed time = 0 ms.

SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 0 ms.

Answer 1

try

SELECT DISTINCT a.SFAccountID, a.SLXID, a.Name 
  FROM [dbo].[Salesforce_Accounts] a WITH(NOLOCK)
  JOIN _SLX_AccountChannel b WITH(NOLOCK)
    ON a.SLXID = b.ACCOUNTID
   AND b.STATUS IN ('Active','Customer', 'Current')
  JOIN [dbo].[Salesforce_Contacts] c WITH(NOLOCK)
    ON a.SFAccountID = c.SFAccountID 
   AND c.Primary__C = 0
  LEFT JOIN [dbo].[Salesforce_Contacts] c2 WITH(NOLOCK) 
    on c2.SFAccountID = a.SFAccountID
   AND c2.Primary__c = 1
 WHERE c2.SFAccountID is null 

Answer 2

as far as i understand it, a not in works in the same fashion as two nested for instructions would.

so, asuming you have two tables: table(1000 records) and tabla (2000 records),

select * from table where table.field not in (select field from tabla)

is like doing

for (int i = 0;  i < 1000; i++) {
   for (int j = 0;  j < 2000; j++) {
   }
}

that is 1000*2000 = 2 million operations.

the left join with tabla.field is null trick, again, as far as i understand it, makes only 2000 operations

Use left join.