You can use either CAST or CONVERT. Because CAST and CONVERT provide similar functionality.

The difference is CASE is more ANSI standard and use this only if you are writing a program which you will need to use in many database platforms. Where CONVERT is Microsoft specific and more powerful.



Links:
http://msdn2.microsoft.com/en-us/library/ms187928.aspx
http://searchwinit.techtarget.com/tip/0,289483,sid1_gci1014050,00.html