(1000 row(s) affected)
Table '#TestData'. Scan count 1001, logical reads 2411, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

(1 row(s) affected)

 SQL Server Execution Times:
   CPU time = 234 ms,  elapsed time = 3175 ms.

(1000 row(s) affected)
Table '#TestData'. Scan count 1001, logical reads 2411, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

(1 row(s) affected)

 SQL Server Execution Times:
   CPU time = 172 ms,  elapsed time = 561 ms.

 WITH CTE AS
(
SELECT DISTINCT 
       AccountNumber
  FROM #TestData
)
SELECT AccountNumber,
       CommaList = STUFF((
                   SELECT ',' + Value
                     FROM #TestData
                    WHERE AccountNumber = CTE.AccountNumber
                    ORDER BY Value
                      FOR XML PATH('')),1,1,'') -- no TYPE.value
  FROM CTE;

WITH CTE (Data) AS
(
SELECT 'Rolling Stones' UNION ALL
SELECT 'Hall & Oates' UNION ALL
SELECT 'One <> Two'
)
SELECT [WithType] = STUFF((
                    SELECT ',' + Data
                      FROM CTE
                       FOR XML PATH(''),TYPE).value('.','varchar(max)'),1,1,''),
       [WithOutType] = STUFF((
                    SELECT ',' + Data
                      FROM CTE
                       FOR XML PATH('')),1,1,'');